[X86] Factor the core code out of LowerSETCC into a helper that can create...

[X86] Factor the core code out of LowerSETCC into a helper that can create CMP/BT/PTEST/KORTEST etc. without making an X86ISD::SETCC node. NFCI

Make each of the helper functions only return their comparison node and the condition code. Leave X86ISD::SETCC creation to the LowerSETCC function itself.

Looking into whether we can use this code directly in BRCOND and SELECT lowering instead of going through LowerSETCC which creates an X86ISD::SETCC node we need to look through.

llvm-svn: 350082