glsl: Lower constant arrays to uniform arrays.

Consider GLSL code such as:

   const ivec2 offsets[] =
      ivec2[](ivec2(-1, -1), ivec2(-1, 0), ivec2(-1, 1),
              ivec2(0, -1),  ivec2(0, 0),  ivec2(0, 1),
              ivec2(1, -1),  ivec2(1, 0),  ivec2(1, 1));

   ivec2 offset = offsets[<non-constant expression>];

Both i965 and nv50 currently handle this very poorly.  On i965, this
becomes a pile of MOVs to load the immediate constants into registers,
a pile of scratch writes to move the whole array to memory, and one
scratch read to actually access the value - effectively the same as if
it were a non-constant array.

We'd much rather upload large blocks of constant data as uniform data,
so drivers can simply upload the data via constbufs, and not have to
populate it via shader instructions.

This is currently non-optional because both i965 and nouveau benefit
from it, and according to Marek radeonsi would benefit today as well.
(According to Tom, radeonsi may want to handle this itself in the long
term, but we can always add a flag when it becomes useful.)

Improves performance in a terrain rendering microbenchmark by about 2x,
and cuts the number of instructions in about half.  Helps a lot of
"Natural Selection 2" shaders, as well as one "HOARD" shader.

total instructions in shared programs: 5473459 -> 5471765 (-0.03%)
instructions in affected programs:     5880 -> 4186 (-28.81%)

v2: Use ir_var_hidden to avoid exposing the new uniform via the GL
    uniform introspection API.

v3: Alphabetize Makefile.sources properly.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77957
Signed-off-by: Kenneth Graunke <[email protected]>

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