[PowerPC] Emit scalar fp min/max instructions

VSX provides floating point minimum and maximum instructions that conform
to IEEE semantics. This legalizes the respective nodes and emits VSX code
for them. Furthermore, on Power9 cores we have xsmaxcdp and xsmincdp
instructions that conform to language semantics for the conditional operator
even in the presence of NaNs.

Differential revision: https://reviews.llvm.org/D62993