glsl: Improve the local dead code optimization to eliminate unused channels.

glsl: Improve the local dead code optimization to eliminate unused channels.

Total instructions: 261582 -> 261316
135/2147 programs affected (6.3%)
36752 -> 36486 instructions in affected programs (0.7% reduction)

This excludes a tropics shader that now gets 16-wide mode and throws
off the numbers.  5 shaders are hurt: two extra MOVs in 4 tropics
shaders it looks like because we don't split register names according
to independent webs, and one gstreamer shader where it looks like
try_rewrite_rhs_to_dst() is falling on its face.

This should also help avoid a regression in VSes from idr's ARB
programs to GLSL work.