nir: Teach nir_opt_algebraic about adding and subtracting the same thing

nir: Teach nir_opt_algebraic about adding and subtracting the same thing

This optimizes a + b - b to just a. Modest shader-db results (BDW):

  total instructions in shared programs: 7842452 -> 7841862 (-0.01%)
  instructions in affected programs:     61938 -> 61348 (-0.95%)
  total loops in shared programs:        2131 -> 2131 (0.00%)
  helped:                                263
  HURT:                                  0
  GAINED:                                0
  LOST:                                  0

but the optimization turns

  gl_VertexID - gl_BaseVertexARB

into just a reference to SYSTEM_VALUE_VERTEX_ID_ZERO_BASE, which the
i965 hardware supports natively. That means we can avoid using the
internal vertex buffer for gl_BaseVertexARB in this case.

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>