[mte] work around lifetime issue with setjmp.

setjmp can return twice, but PostDominatorTree is unaware of this. as
such, it overestimates postdominance, leaving some cases where memory
does not get untagged on return. this causes false positives later in
the program execution.

this is a workaround for now, in the longer term PostDominatorTree
should be made aware of returns_twice, as this may cause problems
elsewhere.

See D118647 for equivalent fix to HWASan.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D118749