R600/SI: use patterns for clamp, fabs, fneg

Instead of using custom inserters, it's simpler and
should make DAG folding easier.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
llvm-svn: 175755