Make it possible for ints/floats to return different values from getBooleanContents()

Summary:
On MIPS32r6/MIPS64r6, floating point comparisons return 0 or -1 but integer
comparisons return 0 or 1.

Updated the various uses of getBooleanContents. Two simplifications had to be
disabled when float and int boolean contents differ:
- ScalarizeVecRes_VSELECT except when the kind of boolean contents is trivially
  discoverable (i.e. when the condition of the VSELECT is a SETCC node).
- visitVSELECT (select C, 0, 1) -> (xor C, 1).
  Come to think of it, this one could test for the common case of 'C'
  being a SETCC too.

Preserved existing behaviour for all other targets and updated the affected
MIPS32r6/MIPS64r6 tests. This also fixes the pi benchmark where the 'low'
variable was counting in the wrong direction because it thought it could simply
add the result of the comparison.

Reviewers: hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, jholewinski, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D4389