Add support for legalizing SETNE/SETEQ by inverting the condition code and the result of the comparison.

Summary:
LegalizeSetCCCondCode can now legalize SETEQ and SETNE by returning the inverse
condition and requesting that the caller invert the result of the condition.

The caller of LegalizeSetCCCondCode must handle the inverted CC, and they do
so as follows:
  SETCC, BR_CC:
    Invert the result of the SETCC with SelectionDAG::getNOT()
  SELECT_CC:
    Swap the true/false operands.

This is necessary for MSA which lacks an integer SETNE instruction.

Reviewers: resistor

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2229