[SimplifyIndVars] Ignore dead users

IndVarSimplify sometimes makes transforms basing on users that are trivially dead. In particular,
if DCE wasn't run before it, there may be a dead `sext/zext` in loop that will trigger widening
transforms, however it makes no sense to do it.

This patch teaches IndVarsSimplify ignore the mist trivial cases of that.

Differential Revision: https://reviews.llvm.org/D47974
Reviewed By: sanjoy

llvm-svn: 334567

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll b/llvm/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll
index 1a63c795548..320f0b30cb9 100644
--- a/llvm/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll
+++ b/llvm/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll
@@ -293,6 +293,8 @@ for.end:                                          ; preds = %if.end, %entry
   ret void
 }
 
+declare void @use(i1 %x)
+
 ; check that we handle conditions with loop invariant operands which
 ; *aren't* in the header - this is a very rare and fragile case where
 ; we have a "loop" which is known to run exactly one iteration but
@@ -320,6 +322,7 @@ latch:
   br i1 true, label %exit, label %loop
 
 exit:
+  call void @use(i1 %cmp)
   ret void
 }