glsl: Avoid extra if statements for logic and/or with no side effects.

glsl: Avoid extra if statements for logic and/or with no side effects.

This avoids extra if statements in the common case of just comparing
two expressions that don't involve assignments or function calls,
along with simplifying the handling of constant expressions.  Reduces
i965 instructions generated in unigine tropics and sanctuary,
yofrankie, warsow, gstreamer shaders, and the weston compositor.

shader-db results:
Total instructions: 213052 -> 212752
38/1246 programs affected (3.0%)
14309 -> 14009 instructions in affected programs (2.1% reduction)