glsl/nir: Add optimization pass for access flags

glsl/nir: Add optimization pass for access flags

Right now, this just deduces when we can arbitrarily reorder SSBO and
image loads, matching the existing logic in radeonsi's TGSI->LLVM pass.
This approach can't handle some things that nir_opt_copy_prop_vars can,
but it can handle images, and with GCM it lets us hoist reads outside of
loops. We can also pass this information to LLVM which lets it do its
own optimizations on it.

This is GLSL only as I haven't tested it on Vulkan yet, and it would
probably need a few changes to work there.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>