Revert of r223763, in spirit.

r223763 was made to work around a temporary issue where a user of the
JIT was passing down a declaration (incorrectly). This shouldn't
occur, so assert rather than silently continue.
Revert "LiveRangeCalc: Rewrite subrange calculation"

Revert until I find out why non-subreg enabled targets break.

This reverts commit 6097277eefb9c5fb35a7f493c783ee1fd1b9d6a7.