LoopDeletion: Move EH pad check before the isLoopNeverExecuted Check (#78189)

This commit modifies `LoopDeletion::deleteLoopIfDead` to check if the
exit block of a loop is an EH pad before checking if the loop gets
executed. This handles the case where an unreachable loop has a
landingpad as an Exit block, and the loop gets deleted, leaving leaving
the landingpad without an edge from an unwind clause.

Fixes #76852.