[PATCH] [X86] 8bit divrem: Improve codegen for AH register extraction.

[Ahmed Bougacha]

Hi lhames, qcolombet,

For 8-bit divrems where the remainder is used, we used to generate:
    divb  %sil
    shrw  $8, %ax
    movzbl  %al, %eax

That was to avoid an H-reg access, which is problematic mainly because
it isn't possible in REX-prefixed instructions.

This patch optimizes that to:
    divb  %sil
    movzbl  %ah, %eax

To do that, we explicitly extend AH, and extract the L-subreg in the
resulting register.  The extension is done using the NOREX variants of
MOVZX.  To support signed operations, MOVSX_NOREX is also added.
Further, this introduces a new SDNode type, [us]divrem_ext_hreg, which is
then lowered to a sequence containing a single zext (rather than 2).

The new node isn't ideal, but I'm not aware of any alternatives to avoid redundant extensions.

http://reviews.llvm.org/D6064

Files:
  lib/Target/X86/X86ISelDAGToDAG.cpp
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  lib/Target/X86/X86InstrExtension.td
  test/CodeGen/X86/divrem8_ext.ll
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