[ValueTracking] remove a FIXME for something we don't want to do; NFC

The comment was added with:
https://reviews.llvm.org/rL293773
...but there would be a cost to implement this and possibly no payoff.

llvm-svn: 293823
diff --git a/llvm/test/Transforms/InstSimplify/assume.ll b/llvm/test/Transforms/InstSimplify/assume.ll
index 2edcc0c..4255238 100644
--- a/llvm/test/Transforms/InstSimplify/assume.ll
+++ b/llvm/test/Transforms/InstSimplify/assume.ll
@@ -28,8 +28,7 @@
 
 ; Similar to above: there's no way to know which assumption is truthful,
 ; so just don't crash. The second icmp+assume gets processed later, so that
-; determines the return value. This can be improved by permanently invalidating
-; the cached assumptions for this function. 
+; determines the return value.
 
 define i8 @conflicting_assumptions(i8 %x) {
 ; CHECK-LABEL: @conflicting_assumptions(