NewGVN: Fix PR31480, PR31483, PR31499, by rewriting how memory congruence handling works.

Summary: Previously, we tried to fix up the equivalences during symbolic evaluation.  This does not work. Now, we change the equivalences during congruence finding, where it belongs.  We also initialize the equivalence table to give a maximal answer.

Reviewers: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28192

llvm-svn: 290815