[WebAssembly] Remove infinite loop from reg-stackify test

r310940 exposed reverse-unreachable code to some optimizers,
which caused some of the code in this test to be sunk, changing
the input to the pass and breaking the exptectations.

Since that change is irrelevant to this particular test, this change
just adds an exit node to work around the problem; the
test should really be more robust (or be an MIR test?) but this preserves
the existing test intent.

llvm-svn: 310981