[SeparateConstOffsetFromGEP] Check correct index for non-negativity

We were checking the index of GEP twice, instead of checking both
GEP and PtrGEP.

Author: nikic

llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp

@@ -1001,7 +1001,7 @@ bool SeparateConstOffsetFromGEP::reorderGEP(GetElementPtrInst *GEP,
     auto KnownGEPIdx = computeKnownBits(GEPIdx->get(), *DL);
     IsChainInBounds &= KnownGEPIdx.isNonNegative();
     if (IsChainInBounds) {
-      auto PtrGEPIdx = GEP->indices().begin();
+      auto PtrGEPIdx = PtrGEP->indices().begin();
       auto KnownPtrGEPIdx = computeKnownBits(PtrGEPIdx->get(), *DL);
       IsChainInBounds &= KnownPtrGEPIdx.isNonNegative();
     }

llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/lower-gep-reorder.ll

@@ -80,8 +80,8 @@ define void @inboundsPossiblyNegative2(ptr %in.ptr, i64 %in.idx1) {
 ; CHECK-LABEL: define void @inboundsPossiblyNegative2(
 ; CHECK-SAME: ptr [[IN_PTR:%.*]], i64 [[IN_IDX1:%.*]]) {
 ; CHECK-NEXT:    [[IN_IDX1_NNEG:%.*]] = and i64 [[IN_IDX1]], 9223372036854775807
-; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds <2 x i8>, ptr [[IN_PTR]], i64 [[IN_IDX1_NNEG]]
-; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds <2 x i8>, ptr [[TMP0]], i64 -1
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr <2 x i8>, ptr [[IN_PTR]], i64 [[IN_IDX1_NNEG]]
+; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr <2 x i8>, ptr [[TMP1]], i64 -1
 ; CHECK-NEXT:    ret void
 ;
   %in.idx1.nneg = and i64 %in.idx1, 9223372036854775807