[PATCH] [X86] Tune LEA usage for Silvermont

[Alexey Volkov]

Hi nadav,

According to Intel Software Optimization Manual on Silvermont in some cases LEA is better to be replaced with ADD instructions:
"The rule of thumb for ADDs and LEAs is that it is justified to use LEA with a valid index and/or displacement for non-destructive destination purposes (especially useful for stack offset cases), or to use a SCALE. Otherwise, ADD(s) are preferable."

Attached patch replaces such LEAs with ADD instructions.

http://reviews.llvm.org/D3826

Files:
  lib/Target/X86/X86.td
  lib/Target/X86/X86FixupLEAs.cpp
  lib/Target/X86/X86InstrInfo.cpp
  lib/Target/X86/X86InstrInfo.h
  lib/Target/X86/X86Subtarget.cpp
  lib/Target/X86/X86Subtarget.h
  lib/Target/X86/X86TargetMachine.cpp
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