i965/fs: Set execution controls explicitly in opt_peephole_sel().

i965/fs: Set execution controls explicitly in opt_peephole_sel().

Emit the SELs and MOVs with the same execution controls as the
original MOVs, and the CMP with the same execution controls as the IF.
Also explicitly check that the execution controls of any pair of MOVs
being folded into a SEL are compatible (which is almost always going
to be the case), since otherwise it would seem wrong to initialize the
builder object below from the then_mov instruction only.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>