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[MergeICmps] Make sorting strongly stable on the rhs.

Description

[MergeICmps] Make sorting strongly stable on the rhs.

Summary:
Because the sort order was not strongly stable on the RHS, whether the
chain could merge would depend on the order of the blocks in the Phi.

EXPENSIVE_CHECKS would shuffle the blocks before sorting, resulting in
non-deterministic merging.

Reviewers: gchatelet

Subscribers: hiraditya, llvm-commits, RKSimon

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62193