[Local] Do not introduce a new `llvm.trap` before `unreachable`

This is the second attempt to remove the `llvm.trap` insertion after
https://reviews.llvm.org/rGe14e7bc4b889dfaffb7180d176a03311df2d4ae6
reverted the first one. It is not clear what the exact issue was back
then and it might already be gone by now, it has been >5 years after
all.

Replaces D106299.

Differential Revision: https://reviews.llvm.org/D106308