[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

lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp

@@ -145,6 +145,9 @@ static void PlaceBlockMarker(
            std::prev(InsertPos)->getOpcode() != WebAssembly::END_LOOP)
       --InsertPos;
   }
+  // The header block in which a 'block' mark will be inserted should have a
+  // terminator because it is branching to a non-layout successor.
+  assert(InsertPos != Header->end());
 
   // Add the BLOCK.
   MachineInstr *Begin =