vc4: Drop some confused code about pack/unpack handling.
vc4: Drop some confused code about pack/unpack handling.

At one point I thought packs and unpacks were in the same field of the
instruction.  They aren't.  These instructions therefore never cause a
pack.

total instructions in shared programs: 89472 -> 89390 (-0.09%)
instructions in affected programs:     15261 -> 15179 (-0.54%)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler