[MergeICmps] Make base ordering really deterministic.

Summary:
The idea is that we now manipulate bases through a `unsigned BaseID` based on
order of appearance in the comparison chain rather than through the `Value*`.

Fixes 40714.

Reviewers: gchatelet

Subscribers: mgrang, jfb, jdoerfert, llvm-commits, hans

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58274

llvm-svn: 354131