i965/vec4: Consider sources of non-GRF-dst instructions for dead channels.

Previously we'd ignore the sources of instructions with non-GRF
destinations when calculating calculating the dead channels. This would
lead to us incorrectly removing the first instruction in this sequence:

   mov vgrf11, ...
   cmp.ne.f0 null, vgrf11, 1.0
   mov vgrf11, ...

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76616
1 file changed