nir: Use a bitfield for image access qualifiers

nir: Use a bitfield for image access qualifiers

This commit expands the current memory access enum to contain the extra
two bits provided for images.  We choose to follow the SPIR-V convention
of NonReadable and NonWriteable because readonly implies that you *can*
read so readonly + writeonly doesn't make as much sense as NonReadable +
NonWriteable.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>