[llvm] [WebAssembly] Avoid same-block dominance walks in RegStackify (NFCI) (PR #218369)

[Frank Denis via llvm-commits]

jedisct1 wrote:

On my machine with the https://github.com/dip-proto/rust-wasm-debug-superslow-compile-time test case this change alone reduces RegStackify time from 46s to 41s.

https://github.com/llvm/llvm-project/pull/218369