Fix a .DEFINITE code-gen bug.

Should not blindly re-use registers that may belong to locals.
First pass at enabling lexical => local lowering.

Needs to take quite a bit of care, due to binder failover. This gets
us through the build and passing the sanity tests. Quite a bunch of
spectest fails, probably due to failures in the analysis.
Don't lower dynamic variables.
Avoid re-using register when deconting.

The input register may actually contain the actual variable, now we do
lexical to local lowering.
Fill out full set of poison calls.

Resolves a great many of the test failures in lex2loc.