[EarlyCSE] Fix hashing of self-compares

Summary:
Update compare normalization in SimpleValue hashing to break ties (when
the same value is being compared to itself) by switching to the swapped
predicate if it has a lower numerical value.  This brings the hashing in
line with isEqual, which already recognizes the self-compares with
swapped predicates as equal.

Fixes PR 42280.

Reviewers: spatel, efriedma, nikic, fhahn, uabelho

Reviewed By: nikic

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63349

llvm-svn: 363598