[WebAssembly] Misc. refactoring in CFGStackify (NFC)

Updating `ScopeTops` is something we frequently do in CFGStackify, so
this factors it out as a function. This also makes a few utility
functions templated so that they are not dependent on input vector
types and simplifies function parameters.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D94046