[PowerPC] FP compare and test XL compat builtins.

[PowerPC] FP compare and test XL compat builtins.

This patch is in a series of patches to provide builtins for
compatability with the XL compiler. This patch adds builtins for compare
exponent and test data class operations on floating point values.

Reviewed By: #powerpc, lei

Differential Revision: https://reviews.llvm.org/D109437