add X86-specific DAG optimization to simplify boolean test

- if a boolean test (X86ISD::CMP or X86ISD:SUB) checks a boolean value
  generated from X86ISD::SETCC, try to simplify the boolean value
  generation and checking by reusing the original EFLAGS with proper
  condition code
- add hooks to X86 specific SETCC/BRCOND/CMOV, the major 3 places
  consuming EFLAGS

part of patches fixing PR12312

llvm-svn: 161687