glsl: Eliminate unused built-in variables after compilation

After compilation (and before linking) we can eliminate quite a few
built-in variables.  Basically, any uniform or constant (e.g.,
gl_MaxVertexTextureImageUnits) that isn't used (with one exception) can
be eliminated.  System values, vertex shader inputs (with one
exception), and fragment shader outputs that are not used and not
re-declared in the shader text can also be removed.

gl_ModelViewProjectMatrix and gl_Vertex are used by the built-in
function ftransform.  There are some complications with eliminating
these variables (see the comment in the patch), so they are not
eliminated.

Valgrind massif results for a trimmed apitrace of dota2:

                  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
Before (32-bit): 46 40,661,487,174       75,116,800       68,854,065     6,262,735            0
After  (32-bit): 50 40,564,927,443       69,185,408       63,683,871     5,501,537            0

Before (64-bit): 64 37,200,329,700      104,872,672       96,514,546     8,358,126            0
After  (64-bit): 59 36,822,048,449       96,526,888       89,113,000     7,413,888            0

A real savings of 4.9MiB on 32-bit and 7.0MiB on 64-bit.

v2: Don't remove any built-in with Transpose in the name.

v3: Fix comment typo noticed by Anuj.

Signed-off-by: Ian Romanick <[email protected]>
Suggested-by: Eric Anholt <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Acked-by: Anuj Phogat <[email protected]>
Cc: Eric Anholt <[email protected]>

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