vc4: Drop integer multiplies with 0 to moves of 0.
vc4: Drop integer multiplies with 0 to moves of 0.

This cleans up more instructions generated by uniform array indexing
multiplies.

total instructions in shared programs: 39989 -> 39961 (-0.07%)
instructions in affected programs:     896 -> 868 (-3.12%)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler