[WebAssembly] Error out when block/loop markers mismatch

Summary:
Currently InstPrinter ignores if there are mismatches between block/loop
and end markers by skipping the case if ControlFlowStack is empty. I
guess it is better to explicitly error out in this case, because this
signals invalid input.

Reviewers: aardappel

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D53620

llvm-svn: 345333