[WebAssembly] Ignore DBG_VALUE in WebAssemblyCFGStackify pass when looking for block start

Summary:
Fixes https://bugs.llvm.org/show_bug.cgi?id=39158 and regression caused by
D49034. Though it is possible the problem was existed before and was exposed by
additional DBG_VALUEs.

Reviewers: sunfish, dschuff, aheejin

Reviewed By: aheejin

Subscribers: sbc100, aheejin, llvm-commits, alexcrichton, jgravelle-google

Differential Revision: https://reviews.llvm.org/D52837

llvm-svn: 343827