[Hexagon] Use stable sort for HexagonShuffler to remove non-deterministic ordering

Summary:
This fixes failures in the following tests uncovered by D39245:
        LLVM :: CodeGen/Hexagon/args.ll
        LLVM :: CodeGen/Hexagon/constp-extract.ll
        LLVM :: CodeGen/Hexagon/expand-condsets-basic.ll
        LLVM :: CodeGen/Hexagon/gp-rel.ll
        LLVM :: CodeGen/Hexagon/packetize_cond_inst.ll
        LLVM :: CodeGen/Hexagon/simple_addend.ll
        LLVM :: CodeGen/Hexagon/swp-stages4.ll
        LLVM :: CodeGen/Hexagon/swp-vmult.ll
        LLVM :: CodeGen/Hexagon/swp-vsum.ll
        LLVM :: MC/Hexagon/align.s
        LLVM :: MC/Hexagon/asmMap.s
        LLVM :: MC/Hexagon/dis-duplex-p0.s
        LLVM :: MC/Hexagon/double-vector-producer.s
        LLVM :: MC/Hexagon/inst_select.ll
        LLVM :: MC/Hexagon/instructions/j.s

Reviewers: colinl, kparzysz, adasgupt, slarin

Reviewed By: kparzysz

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40227

llvm-svn: 319223