Further "calculate current Bdescr" fixes.

When no allocation occurs on a branch, we can cause an off-by-one
error (this is detectable when Hp == HpLim). Now, HpLim is always
guaranteed to be in bounds, so use that.  Probably a better way is
to just keep track of the current Bdescr in a register.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>