R600/SI: Custom lower i1 stores

These are sometimes created by the shrink to boolean optimization in the
globalopt pass.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
R600/SI: Using SGPRs is illegal for instructions that read carry-out from VCC

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>