vc4: Don't try to CSE non-SSA instructions.
vc4: Don't try to CSE non-SSA instructions.

This can happen when we're doing destination packing -- we don't know
what's in the rest of the register.

Signed-off-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler