[SCEV] Add support for x == constant to CollectCondition.

Add support for EQ predicates with constant operand. In that case, using
the constant instead of an unknown expression should always be
beneficial.

Author: fhahn

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -12618,6 +12618,10 @@ const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
       }
       break;
     }
+    case CmpInst::ICMP_EQ:
+      if (isa<SCEVConstant>(RHS))
+        RewriteMap[LHSUnknown->getValue()] = RHS;
+      break;
     default:
       break;
     }

llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll

@@ -94,7 +94,7 @@ exit:
 define void @test_guard_eq_12(i32* nocapture %a, i64 %N) {
 ; CHECK-LABEL: Determining loop execution counts for: @test_guard_eq_12
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is %N
-; CHECK-NEXT:  Loop %loop: max backedge-taken count is -1
+; CHECK-NEXT:  Loop %loop: max backedge-taken count is 12
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is %N
 ;
 entry: