nir: optimize pack_uint_2x16 of pack_half(a, 0)
Foz-DB Navi31:
Totals from 31 (0.04% of 79395) affected shaders:
Instrs: 6157 -> 6065 (-1.49%)
CodeSize: 35676 -> 34936 (-2.07%)
Latency: 23979 -> 23805 (-0.73%); split: -0.79%, +0.07%
InvThroughput: 5248 -> 5124 (-2.36%)
VALU: 3224 -> 3162 (-1.92%)