R600/SI: fix ELSE pseudo op handling

Restore the EXEC mask early, otherwise a copy might end up not beeing executed.

Candidate for the mesa stable branch.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
llvm-svn: 178018