nir/opt_move: fix handling of if-condition

nir/opt_move: fix handling of if-condition

By accident, this used the parent of the nir_src which is a nir_if
instead of the parent of the SSA value.

Totals from 10814 (8.10% of 133461) affected shaders: (GFX11)
Instrs: 21759185 -> 21757190 (-0.01%); split: -0.02%, +0.01%
CodeSize: 112320272 -> 112316008 (-0.00%); split: -0.02%, +0.01%
SpillSGPRs: 11220 -> 11212 (-0.07%)
SpillVGPRs: 911 -> 903 (-0.88%); split: -1.54%, +0.66%
Latency: 258334759 -> 258316073 (-0.01%); split: -0.02%, +0.01%
InvThroughput: 31428650 -> 31426394 (-0.01%); split: -0.02%, +0.01%
VClause: 309119 -> 309090 (-0.01%); split: -0.01%, +0.01%
SClause: 657028 -> 657150 (+0.02%); split: -0.03%, +0.04%
Copies: 1434209 -> 1432420 (-0.12%); split: -0.28%, +0.15%
Branches: 481804 -> 481801 (-0.00%)
PreSGPRs: 829995 -> 829966 (-0.00%)
PreVGPRs: 758249 -> 758253 (+0.00%)

Fixes: 8a78706643ecad8a1f303cc9358873abc29978b4 ('nir: refactor nir_opt_move')
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24695>