Instead of generating GC maps which describe which stack locations that
are uninitialized, we emit ##clear instructions for those locations in
front of ##call-gc instructions. This makes the context root scanning
much simpler because the GC can assume that all stack slots are
initialized. It also removes the compiler.cfg.stacks.vacant pass and
seem to reduce the image size slightly because many fewer GC maps needs
to be emitted.
the padding vocab is inverted so that instead of trying to track which
locations are live, it tracks those which are dead which is much easier
to get right.
this step is supposed to analyze the cfg, find where there are peeks
that can cause stack underflow, and insert replace-imm instructions so
that there are no uninitialized stack locations if they do. could fix#1187
Björn Lindqvist