nir/peephole_select: Allow uniform/input loads and load_const

nir/peephole_select: Allow uniform/input loads and load_const

Shader-db results on HSW:

total instructions in shared programs: 4174156 -> 4157291 (-0.40%)
instructions in affected programs:     145397 -> 128532 (-11.60%)
helped:                                383
HURT:                                  0
GAINED:                                20
LOST:                                  22

There are two more tests lost than gained.  However, comparing this with
GLSL IR vs. NIR results, the overall delta is reduced from 85/44
gained/lost on current master to 71/32 with this commit.  Therefore, I
think it's probably a boon since we are getting "closer" to where we were
before.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>