[GVN] Don't replace constants with constants.

This fixes PR34908. Patch by Alex Crichton!

Differential Revision:  https://reviews.llvm.org/D38765

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315429 91177308-0d34-0410-b5e6-96231b3b80d8

Author: dcci

lib/Transforms/Scalar/GVN.cpp

@@ -1747,6 +1747,11 @@ bool GVN::processAssumeIntrinsic(IntrinsicInst *IntrinsicI) {
     }
     markInstructionForDeletion(IntrinsicI);
     return false;
+  } else if (isa<Constant>(V)) {
+    // If it's not false, and constant, it must evaluate to true. This means our
+    // assume is assume(true), and thus, pointless, and we don't want to do
+    // anything more here.
+    return false;
   }
 
   Constant *True = ConstantInt::getTrue(V->getContext());

test/Transforms/GVN/pr34908.ll

@@ -0,0 +1,13 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -gvn -S | FileCheck %s
+
+define i1 @foo() {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    call void @llvm.assume(i1 undef)
+; CHECK-NEXT:    ret i1 undef
+;
+  call void @llvm.assume(i1 undef)
+  ret i1 undef
+}
+
+declare void @llvm.assume(i1)