Avoid emitting unreachable SP adjustments after `throw`

In 172eee9c, we tried to avoid these by modelling the callee as
internally resetting the stack pointer.

However, for the majority of functions with reserved stack frames, this
would lead LLVM to emit extra SP adjustments to undo the callee's
internal adjustment. This lead us to fix the problem further on down the
pipeline in eliminateCallFramePseudoInstr. In 5b79e603, I added
use a heuristic to try to detect when the adjustment would be
unreachable.

This heuristic is imperfect, and when exception handling is involved, it
fails to fire. The new test is an example of this. Simply throwing an
exception with an active cleanup emits dead SP adjustments after the
throw. Not only are they dead, but if they were executed, they would be
incorrect, so they are confusing.

This change essentially reverts 172eee9c and makes the 5b79e603
heuristic responsible for preventing unreachable stack adjustments. This
means we may emit unreachable stac...