[WebAssembly] Add an assertion for an invalid CFG
It was not easy to provide a test case for D45648 (rL330079) because the bug
didn't manifest itself in the set of currently valid IRs. Added an assertion to
check this faster, thanks to @dblaikie's suggestion.
Subscribers: jfb, dschuff, sbc100, jgravelle-google, llvm-commits, dblaikie
Differential Revision: https://reviews.llvm.org/D45711