When performing common subexpression elimination on instructions with
non-null destinations we emit a MOV to copy the result to a new
register that must have no other uses. In the case of:

   cmp.g.f0.0(8) null:D, vgrf43:F, 0.500000f
   ...
   cmp.g.f0.0(8) vgrf113:D, vgrf43:F, 0.500000f

we put the first instruction in the AEB and decided that we could reuse
its result when we found the second. Unfortunately, that meant that we'd
emit a MOV from the first's destination, which is null.

Don't do anything if the entry's destination is null and the
instruction's destination is non-null.

Tested-by: Tapani Pälli <tapani.palli@intel.com>