[ValueTracking] Early exit when further analysis won't be fruitful.

This should have NFC in the context of codegen, but may have positive
implications on compile-time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268651 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed