[WebAssembly] Fix updating/moving DBG_VALUEs in RegStackify

Summary:
As described in PR40209, there can be issues in DBG_VALUEs handling when multiple defs present in a BB. This patch
adds logic for detection of related to def DBG_VALUEs and localizes register update and movement to found DBG_VALUEs.

Reviewers: aheejin

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

Differential Revision: https://reviews.llvm.org/D56401

llvm-svn: 351216