nir: Try commutative sources in CSE

nir: Try commutative sources in CSE

Shader-db results:

GM45 NIR:
total instructions in shared programs: 4082044 -> 4081919 (-0.00%)
instructions in affected programs:     27609 -> 27484 (-0.45%)
helped:                                44

Iron Lake NIR:
total instructions in shared programs: 5678776 -> 5678646 (-0.00%)
instructions in affected programs:     27406 -> 27276 (-0.47%)
helped:                                45

Sandy Bridge NIR:
total instructions in shared programs: 7329995 -> 7329096 (-0.01%)
instructions in affected programs:     142035 -> 141136 (-0.63%)
helped:                                406
HURT:                                  19

Ivy Bridge NIR:
total instructions in shared programs: 6769314 -> 6768359 (-0.01%)
instructions in affected programs:     140820 -> 139865 (-0.68%)
helped:                                423
HURT:                                  2

Haswell NIR:
total instructions in shared programs: 6183693 -> 6183298 (-0.01%)
instructions in affected programs:     96538 -> 96143 (-0.41%)
helped:                                303
HURT:                                  4

Broadwell NIR:
total instructions in shared programs: 7501711 -> 7498170 (-0.05%)
instructions in affected programs:     266403 -> 262862 (-1.33%)
helped:                                705
HURT:                                  5
GAINED:                                4

v2: Rebase on top of Connor's fix.

v3: Convert the if-test for num_inputs == 2 to an assertion.  Suggested
by Jason after some comments / questions by Ilia.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com> [v1]
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: Connor Abbott <cwabbott0@gmail.com>