nir: Collapse repeated bcsels on the same argument.
nir: Collapse repeated bcsels on the same argument.

vc4 results:
total instructions in shared programs: 39881 -> 39794 (-0.22%)
instructions in affected programs:     6302 -> 6215 (-1.38%)

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Reviewed-by: Connor Abbott <71178acffcc112b21e5858656e5751f5e4aa9364@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler