glsl: Drop no-op shifts involving 0.
glsl: Drop no-op shifts involving 0.

I noticed this in a shader in Unigine Heaven that was spilling.  While it
doesn't really reduce register pressure, it shaves a few instructions
anyway (7955 -> 7882).

v2: Fix turning "0 >> x" into "x" instead of "0" (caught by Erik
    Faye-Lund).

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

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