[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330217 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed