nir/algebraic: optimize b<<a + c<<a

nir/algebraic: optimize b<<a + c<<a

fossil-db (navi21):
Totals from 248 (0.18% of 135636) affected shaders:
Instrs: 85836 -> 85611 (-0.26%); split: -0.27%, +0.00%
CodeSize: 481304 -> 480332 (-0.20%); split: -0.21%, +0.00%
Latency: 9596559 -> 9596152 (-0.00%); split: -0.00%, +0.00%
InvThroughput: 1423707 -> 1423670 (-0.00%)
SClause: 3872 -> 3874 (+0.05%)
PreSGPRs: 5034 -> 5038 (+0.08%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19137>