nvc0/ir: detect i2f/i2i which operate on specific bytes/words

nvc0/ir: detect i2f/i2i which operate on specific bytes/words

Some Unigine shaders have been observed to unpack bytes out of 32-bit
integers and convert them to floats. I2F/I2I can handle this sort of
thing directly. Detect the handleable situations.

This misses 16-bit word capabilities in nv50, but I haven't seen shaders
that would actually make use of that.

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