i965/fs: Don't perform CSE on inst HW_REG dests (unless it's null)

i965/fs: Don't perform CSE on inst HW_REG dests (unless it's null)

Commit b16b3c87 began performing CSE on CMP instructions with null
destinations. I relaxed the restrictions a bit too much, thereby
allowing CSE to be performed on instructions with, for instance, an
explicit accumulator destination.

This broke the arb_gpu_shader5/fs-imulExtended shader tests because
they emit MUL instructions with the accumulator as the destination. CSE
would instead cause the MUL to write to a GRF, which is lower precision
than the accumulator.

Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: 10.0 <mesa-stable@lists.freedesktop.org>