Auto merge of #119439 - cjgillot:gvn-faster, r=oli-obk

Avoid some redundant work in GVN

The first 2 commits are about reducing the perf effect.

Third commit avoids doing redundant work: is a local is SSA, it already has been simplified, and the resulting value is in `self.locals`. No need to call any code on it.

The last commit avoids removing some storage statements.

r? wg-mir-opt

Author: bors