[AMDGPU] Fix atomic float max/min intrinsics

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