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
Florian Hahn