i965/fs: Simplify interference scan in register coalescing.

i965/fs: Simplify interference scan in register coalescing.

We were starting at the beginning of the instruction list, rather than
with the MOV instruction itself. This allows us to coalesce after
control flow.

Excluding the shaders from an unreleased title, the shader-db results:

total instructions in shared programs: 1603791 -> 1594215 (-0.60%)
instructions in affected programs:     678772 -> 669196 (-1.41%)
GAINED:                                5
LOST:                                  0

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>