glsl: Track initial mask in constant propagation live set

glsl: Track initial mask in constant propagation live set

The set of values initially available (before any kills) must be
tracked with each constant in the set.  Otherwise the wrong component
can be selected after earlier components have been killed.

NOTE: This is a candidate for the 7.10 and 7.11 branches.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=37383
Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Matthias Bentrup <matthias.bentrup@googlemail.com>