Hooked up raw.buffer.atomic.fmin/max.f64
This instruction should be available on GFX6, GFX7, and GFX10.
It was implemented for GFX90a with a different name.
Added intrinsic def for image_atomic_fmin/fmax; the instruction
defs were already there.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D108208
Change-Id: I473f98d28b2afbeeb2c27822d9686b5e86634e2f
Joe Nash