glsl: Remove optimize_swizzles.
It will get turned into SSA and copy-propagated in NIR, no need to walk the IR collapsing it here.
iris shader-db results appear to be noise:
total instructions in shared programs: 8932195 -> 8932147 (<.01%)
instructions in affected programs: 537 -> 489 (-8.94%)
LOST: 12
GAINED: 11
lost/gained are simd32 switches in unigine, l4d2, portal2, asphalt9.
Edited by Emma Anholt