R600: control flow optimization

Branch if we have enough instructions so that it makes sense.
Also remove branches if they don't make sense.

Patch by: Christian König

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Christian König <deathsimple@vodafone.de>
llvm-svn: 170592