[PATCH] [X86] Allow atomic operations using immediates to avoid using a register

[Robin Morisset]

Hi jfb,

The only valid lowering of atomic stores in the X86 backend was mov from
register to memory. As a result, storing an immediate required a useless copy
of the immediate in a register. Now these can be compiled as a simple mov.

Similarily, adding/and-ing/or-ing/xor-ing an
immediate to an atomic location (but through an atomic_store/atomic_load,
not a fetch_whatever intrinsic) can now make use of an 'add $imm, x(%rip)'
instead of using a register. And the same applies to inc/dec.

This second point matches the first issue identified in
  http://llvm.org/bugs/show_bug.cgi?id=17281

http://reviews.llvm.org/D4796

Files:
  lib/Target/X86/X86CodeEmitter.cpp
  lib/Target/X86/X86InstrCompiler.td
  lib/Target/X86/X86MCInstLower.cpp
  test/CodeGen/X86/atomic_mi.ll
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