[PATCH] [X86] Memory folding for commutative instructions (updated)

[Simon Pilgrim]

Hi qcolombet, andreadb, spatel, nadav,

This patch improves support for commutative instructions in the x86 memory folding implementation by attempting to fold a commuted version of the instruction if the original folding fails - if that folding fails as well the instruction is 're-commuted' back to its original order before returning.

Updated version of http://reviews.llvm.org/D5701 (reverted in r219595) - the commutation attempt now explicitly ensures that neither of the commuted source operands are tied to the destination operand / register, which was the source of all the regressions that occurred with the original patch attempt.

Added additional regression test case provided by Joerg Sonnenberger.

http://reviews.llvm.org/D5818

Files:
  lib/Target/X86/X86FastISel.cpp
  lib/Target/X86/X86InstrInfo.cpp
  lib/Target/X86/X86InstrInfo.h
  test/CodeGen/X86/2014-10-16-xxhash.ll
  test/CodeGen/X86/avx1-stack-reload-folding.ll
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