i965/fs: Add support for uniform array access with a variable index.

i965/fs: Add support for uniform array access with a variable index.

Serious Sam 3 had a shader hitting this path, but it's used rarely so it
didn't show a significant performance difference (n=7).  It does reduce
compile time massively, though -- one shader goes from 14s compile time
and 11723 instructions generated to .44s and 499 instructions.

Note that some shaders lose 16-wide mode because we don't support
16-wide and pull constants at the moment (generally, things looping over
a few-element array where the loop isn't getting unrolled).  Given that
those shaders are being generated with 15-20% fewer instructions, it
probably outweighs the loss of 16-wide.