nir/lower_variables: Follow the Cytron paper more closely

nir/lower_variables: Follow the Cytron paper more closely

Previously, our variable renaming algorithm, while similar to the one in
the Cytron paper, was not the same.  While I'm pretty sure it was correct,
it will be easier for readers of the code in the variable renaming pass if
it follows more closely.  This commit removes the automatic stack popping
we were doing and replaces it with explicit popping like Cytron does.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>