mesa: Add partial constant propagation pass for Mesa IR

mesa: Add partial constant propagation pass for Mesa IR

This cleans up some code generated by the IR-to-Mesa pass for i915.
In particular, some shaders involving arrays of constant matrices
result in really bad code.

v2: Silence several warnings from merging the gl_constant_value work.
Fix DP[23] folding.  Add support for a bunch more opcodes that appear
in piglit runs on i915.

Reviewed-by: Eric Anholt <eric@anholt.net>