[Clang] Add min/max reduction builtins.

This patch implements __builtin_reduce_max and __builtin_reduce_min as
specified in D111529.

The order of operations does not matter for min or max reductions and
they can be directly lowered to the corresponding
llvm.vector.reduce.{fmin,fmax,umin,umax,smin,smax} intrinsic calls.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D112001