[MergeICmps] Make sorting strongly stable on the rhs.
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
Subscribers: hiraditya, llvm-commits, RKSimon
Differential Revision: https://reviews.llvm.org/D62193