[WebAssembly] Add an assertion for an invalid CFG

Summary:
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.

Reviewers: dblaikie

Subscribers: jfb, dschuff, sbc100, jgravelle-google, llvm-commits, dblaikie

Differential Revision: https://reviews.llvm.org/D45711

llvm-svn: 330217