i965/fs: Add a local common subexpression elimination pass.

i965/fs: Add a local common subexpression elimination pass.

Total instructions: 18210 -> 17836
49/163 programs affected (30.1%)
12888 -> 12514 instructions in affected programs (2.9% reduction)

This reduces Lightsmark's "Scale down filter" shader from 395
instructions to 283, a whopping 28%.  It also reduces register pressure
significantly: the SIMD8 program now uses 29 registers instead of 101,
giving us more than enough room for a SIMD16 program.

v2: Add && !inst->conditional_mod to the "skip some instructions" check.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>