[NewGVN] Don't incorrectly reset the memory leader.

This code was missing a check for stores, so we were thinking the
congruency class didn't have any memory members, and reset the
memory leader.

Differential Revision:  https://reviews.llvm.org/D33056

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302905 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed