[Clang][PowerPC] Add max/min intrinsics to Clang and PPC backend

Add support for builtin_[max|min] which has below prototype:
A builtin_max (A1, A2, A3, ...)
All arguments must have the same type; they must all be float, double, or long double.
Internally use SelectCC to get the result.

Reviewed By: qiucf

Differential Revision: https://reviews.llvm.org/D122478