[CodeGenPrepare] Mend "avoid crashing from replacing a phi twice" fix.

Summary:
An erroneously negated if-statement by an earlier (March 2019) bugfix left phi replacement/simplification under optimizeMemoryInst()  in CodeGenPrepare largely inactivated. The error was found when csmith found that the same assert as in the original bug report could still be triggered in a different way. This patch fixes the bugfix. The original bug was:
 https://bugs.llvm.org/show_bug.cgi?id=41052
... and the previous fix was D59358.

Reviewers: aprantl, skatkov

Reviewed By: skatkov

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67838

llvm-svn: 373084