Use an integer constant for FABS / FNEG (x86).

This change will ease refactoring LowerFABS() and LowerFNEG()
since they have a lot of overlap.

Remove the creation of a floating point constant from an integer
because it's going to be used for a bitwise integer op anyway.

No change to codegen expected, but the verbose comment string
for asm output may change from float values to hex (integer),
depending on whether the constant already exists or not.

Differential Revision: http://reviews.llvm.org/D5052