nir: Implement CSE on intrinsics that can be eliminated and reordered.

nir: Implement CSE on intrinsics that can be eliminated and reordered.

Matt and I noticed that one of the shaders hurt by INTEL_USE_NIR=1 had
load_input and load_uniform intrinsics repeated several times, with the
same parameters, but each one generating a distinct SSA value.  This
made ALU operations on those values appear distinct as well.

Generating distinct SSA values is silly - these are read only variables.
CSE'ing them makes everything use a single SSA value, which then allows
other operations to be CSE'd away as well.

Generalizing a bit, it seems like we should be able to safely CSE any
intrinsics that can be eliminated and reordered.  I didn't implement
support for variables for the time being.

v2: Assert that info->num_variables == 0 (requested by Jason).

total NIR instructions in shared programs: 2435936 -> 2023511 (-16.93%)
NIR instructions in affected programs:     2413496 -> 2001071 (-17.09%)
helped:                                    16872

total i965 instructions in shared programs: 6028987 -> 6008427 (-0.34%)
i965 instructions in affected programs:     640654 -> 620094 (-3.21%)
helped:                                     2071
HURT:                                       585
GAINED:                                     14
LOST:                                       25

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>