[InstCombine] Add test for freeze of GEP with recurrence offset (NFC) (#145541)

The freeze should be pushed through the GEP to the ptr like in:
https://godbolt.org/z/jrcozT8rz

Author: c-rhodes

llvm/test/Transforms/InstCombine/freeze.ll

@@ -877,6 +877,104 @@ exit:                                             ; preds = %loop
   ret void
 }
 
+; The recurrence for the GEP offset can't produce poison so the freeze should
+; be pushed through to the ptr, but this is not currently supported.
+define void @fold_phi_gep_phi_offset(ptr %init, ptr %end, i64 noundef %n) {
+; CHECK-LABEL: @fold_phi_gep_phi_offset(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[I:%.*]] = phi ptr [ [[INIT:%.*]], [[ENTRY:%.*]] ], [ [[I_NEXT_FR:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[OFF:%.*]] = phi i64 [ [[N:%.*]], [[ENTRY]] ], [ [[OFF_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[OFF_NEXT]] = shl i64 [[OFF]], 3
+; CHECK-NEXT:    [[I_NEXT:%.*]] = getelementptr i8, ptr [[I]], i64 [[OFF_NEXT]]
+; CHECK-NEXT:    [[I_NEXT_FR]] = freeze ptr [[I_NEXT]]
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq ptr [[I_NEXT_FR]], [[END:%.*]]
+; CHECK-NEXT:    br i1 [[COND]], label [[LOOP]], label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:                                             ; preds = %loop, %entry
+  %i = phi ptr [ %init, %entry ], [ %i.next.fr, %loop ]
+  %off = phi i64 [ %n, %entry ], [ %off.next, %loop ]
+  %off.next = shl i64 %off, 3
+  %i.next = getelementptr i8, ptr %i, i64 %off.next
+  %i.next.fr = freeze ptr %i.next
+  %cond = icmp eq ptr %i.next.fr, %end
+  br i1 %cond, label %loop, label %exit
+
+exit:                                             ; preds = %loop
+  ret void
+}
+
+; Offset is still guaranteed not to be poison, so the freeze could be moved
+; here if we strip inbounds from the GEP, but this is not currently supported.
+define void @fold_phi_gep_inbounds_phi_offset(ptr %init, ptr %end, i64 noundef %n) {
+; CHECK-LABEL: @fold_phi_gep_inbounds_phi_offset(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[I:%.*]] = phi ptr [ [[INIT:%.*]], [[ENTRY:%.*]] ], [ [[I_NEXT_FR:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[OFF:%.*]] = phi i64 [ [[N:%.*]], [[ENTRY]] ], [ [[OFF_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[OFF_NEXT]] = shl i64 [[OFF]], 3
+; CHECK-NEXT:    [[I_NEXT:%.*]] = getelementptr inbounds i8, ptr [[I]], i64 [[OFF_NEXT]]
+; CHECK-NEXT:    [[I_NEXT_FR]] = freeze ptr [[I_NEXT]]
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq ptr [[I_NEXT_FR]], [[END:%.*]]
+; CHECK-NEXT:    br i1 [[COND]], label [[LOOP]], label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:                                             ; preds = %loop, %entry
+  %i = phi ptr [ %init, %entry ], [ %i.next.fr, %loop ]
+  %off = phi i64 [ %n, %entry ], [ %off.next, %loop ]
+  %off.next = shl i64 %off, 3
+  %i.next = getelementptr inbounds i8, ptr %i, i64 %off.next
+  %i.next.fr = freeze ptr %i.next
+  %cond = icmp eq ptr %i.next.fr, %end
+  br i1 %cond, label %loop, label %exit
+
+exit:                                             ; preds = %loop
+  ret void
+}
+
+; GEP can produce poison, check freeze isn't moved.
+define void @cant_fold_phi_gep_phi_offset(ptr %init, ptr %end, i64 %n) {
+; CHECK-LABEL: @cant_fold_phi_gep_phi_offset(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[I:%.*]] = phi ptr [ [[INIT:%.*]], [[ENTRY:%.*]] ], [ [[I_NEXT_FR:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[OFF:%.*]] = phi i64 [ [[N:%.*]], [[ENTRY]] ], [ [[OFF_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[OFF_NEXT]] = shl i64 [[OFF]], 3
+; CHECK-NEXT:    [[I_NEXT:%.*]] = getelementptr inbounds i8, ptr [[I]], i64 [[OFF_NEXT]]
+; CHECK-NEXT:    [[I_NEXT_FR]] = freeze ptr [[I_NEXT]]
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq ptr [[I_NEXT_FR]], [[END:%.*]]
+; CHECK-NEXT:    br i1 [[COND]], label [[LOOP]], label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:                                             ; preds = %loop, %entry
+  %i = phi ptr [ %init, %entry ], [ %i.next.fr, %loop ]
+  %off = phi i64 [ %n, %entry ], [ %off.next, %loop ]
+  %off.next = shl i64 %off, 3
+  %i.next = getelementptr inbounds i8, ptr %i, i64 %off.next
+  %i.next.fr = freeze ptr %i.next
+  %cond = icmp eq ptr %i.next.fr, %end
+  br i1 %cond, label %loop, label %exit
+
+exit:                                             ; preds = %loop
+  ret void
+}
+
 define void @fold_phi_multiple_insts(i32 %init, i32 %n) {
 ; CHECK-LABEL: @fold_phi_multiple_insts(
 ; CHECK-NEXT:  entry: