R600: Optimize and cleanup KILL on SI

We shouldn't insert KILL optimization if we don't have a
kill instruction at all.

Patch by: Christian König

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Christian König <deathsimple@vodafone.de>
llvm-svn: 172845