vc4: Add support for turning add-based MOVs to muls for pairing.
vc4: Add support for turning add-based MOVs to muls for pairing.

total instructions in shared programs: 43053 -> 40795 (-5.24%)
instructions in affected programs:     37996 -> 35738 (-5.94%)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler