[WebAssembly] Split EH MIR tests into two files

Currently exception.mir runs LateEHPrepare and CFGStackify, but some
tests I'm planning to add shouldn't be run with LateEHPrepare, because
it is convenient to only run CFGStackify when testing things like unwind
mismatches and it is easier to add tests that are in phase right before
CFGStackify. This splits existing exception.mir into two files;
cfg-stackify-eh.mir will only run CFGStackify. Note that
`eh_label_tests` tests both LateEHPrepare and CFGStackify, so it is
still in exception.mir. `rethrow_arg_tests` has been converted to the
post-LateEHPrepare form to be moved into cfg-stackify-eh.mir, like
removing `CATCHRET` and such, because it does not really test anything
in LateEHPrepare.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D97175

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