[WebAssembly] Handle END_LOOP in unreachable BB in CFGStackify

Summary:
This fixes crashes when a BB in which an END_LOOP is to be placed is
unreachable and does not have any predecessors. Fixes PR41307.

Reviewers: dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60004

llvm-svn: 357303