R600: Add LDS_MAX_[U]INT[_RET] instructions for Evergreen

This was only present for SI before.

Cayman may still be missing, but I am unable to test that currently.

v2: Don't create atomicrmw max tests in separate file

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Matt Arsenault <matthew.arsenault@amd.com>
CC: Tom Stellard <thomas.stellard@amd.com>
llvm-svn: 217589