mesa: Fix up the remove_dead_code pass to operate on a channel basis.
mesa: Fix up the remove_dead_code pass to operate on a channel basis.

This cleans up a bunch of instructions in GLSL programs to have limited
writemasks, which would translate to wins in shaders that hit the i965
brw_wm_glsl.c path by depending less on in-driver optimizations.  It will
also help hit other optimization passes I'm looking at.

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