[release/11.0] [mono][llvm] Properly link endfinally to leave targets - #133242

When emitting a leave instruction, we need to find all finally blocks so
that we call all those handlers beforehand. LLVM chooses to link the
finally block with the leave targets so it does it and this point. It
does it however in a wrong way, by linking the bblock containing the
last instruction to the leave target. This is incorrect because a
finally block can have multiple endfinally opcodes and the last opcode
doesn't necessarily need to be an endfinally in the first place. This
invalid CFG generation could trigger asserts in later optimization
passes.

As a fix, whenever we compile a leave and obtain all the protecting
finally handlers, we store them in a per-finally handler list. Then,
once the code gen is finished, we iterate over all instructions of
bblocks in those finally blocks and, whenever we encounter an
endfinally, we link that bblock to the leave target. This fixes the
invalid linking of bblocks as well as properly handling the scenario of
having multiple endfinally blocks.

Fixes #132936