nir: optimize i2f(f2i(fsign))

nir: optimize i2f(f2i(fsign))

Foz-DB Navi10:
Totals from 3013 (2.23% of 134906) affected shaders:
VGPRs: 138068 -> 136964 (-0.80%); split: -0.80%, +0.00%
CodeSize: 10476416 -> 10391800 (-0.81%)
MaxWaves: 79118 -> 80088 (+1.23%)
Instrs: 1963227 -> 1945003 (-0.93%)
Latency: 24734883 -> 24649279 (-0.35%); split: -0.39%, +0.05%
InvThroughput: 6366777 -> 6334735 (-0.50%); split: -0.50%, +0.00%
VClause: 36845 -> 36882 (+0.10%); split: -0.26%, +0.36%
SClause: 59249 -> 59273 (+0.04%); split: -0.25%, +0.29%
Copies: 108570 -> 108501 (-0.06%); split: -0.19%, +0.13%
PreSGPRs: 105371 -> 105862 (+0.47%)
PreVGPRs: 117675 -> 116625 (-0.89%); split: -0.89%, +0.00%

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22003>