[PowerPC] Add custom lowering for SELECT_CC fp128 using xsmaxcqp

Power ISA 3.1 adds xsmaxcqp/xsmincqp for quad-precision type-c max/min selection,
and this opens the opportunity to improve instruction selection on: llvm.maxnum.f128,
llvm.minnum.f128, and select_cc ordered gt/lt and (don't care) gt/lt.

Reviewed By: nemanjai, shchenz, amyk

Differential Revision: https://reviews.llvm.org/D117006