nir/opt_algebraic: optimize unpack_half_2x16_split_x(ushr, a, 16)

nir/opt_algebraic: optimize unpack_half_2x16_split_x(ushr, a, 16)

Same as extract_u16(a, 1)

Totals from 2021 (1.48% of 136546) affected shaders (RAVEN):
VGPRs: 129516 -> 129524 (+0.01%); split: -0.00%, +0.01%
CodeSize: 12485704 -> 12486600 (+0.01%); split: -0.00%, +0.01%
Instrs: 2435041 -> 2434999 (-0.00%); split: -0.00%, +0.00%
Cycles: 20952552 -> 20952624 (+0.00%); split: -0.00%, +0.00%
VMEM: 374492 -> 374212 (-0.07%); split: +0.01%, -0.08%
SMEM: 123309 -> 123291 (-0.01%); split: +0.00%, -0.02%
VClause: 64156 -> 64164 (+0.01%)
Copies: 191620 -> 191616 (-0.00%); split: -0.03%, +0.03%

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6777>