X86: optimize generated code for integer ABS

This patch will generate the following for integer ABS:
      movl    %edi, %eax
      negl    %eax
      cmovll  %edi, %eax
INSTEAD OF
      movl    %edi, %ecx
      sarl    $31, %ecx
      leal    (%rdi,%rcx), %eax
      xorl    %ecx, %eax

There exists a target-independent DAG combine for integer ABS, which converts
integer ABS to sar+add+xor. For X86, we match this pattern back to neg+cmov. 
This is implemented in PerformXorCombine.

rdar://10695237

llvm-svn: 158175