nir/lower_variables: Add a bunch of comments and re-arrange a few things
nir/lower_variables: Add a bunch of comments and re-arrange a few things

This commit seeks to make the lower_variables pass much more clear by
adding a pile of comments and re-arranging a few things.  There are no
functional or algorithmic changes.

Reviewed-by: Connor Abbott <71178acffcc112b21e5858656e5751f5e4aa9364@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler