AMDGPU/GlobalISel: Add clamp combine

Add clamp combine. Source is fminnum(fmaxnum(Val, 0.0), 1.0) or
fmaxnum(fminnum(Val, 1.0), 0.0) or fmed3 intrinsic with 0.0 and
1.0 as two out of three operands.

Differential Revision: https://reviews.llvm.org/D90052