nv50/ir: reduce degree limit on ops that can't encode large reg dests
nv50/ir: reduce degree limit on ops that can't encode large reg dests

Operations that take immediates can only encode registers up to 64. This
fixes a shader in a "Powered by Unity" intro.

Signed-off-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler