[SimplifyCFG] Skip dbg intrinsics when checking for branch-only BBs.

Debug intrinsics are free to hoist and should be skipped when looking
for terminator-only blocks. As a consequence, we have to delegate to the
main hoisting loop to hoist any dbg intrinsics instead of jumping to the
terminator case directly.

This fixes PR49982.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D100640