Cranelift: Prevent DSE across divergent paths

[subotac]

Fixes #14053.

Cranelift's post-dominator tree only considers paths that reach an explicit CFG exit. Dead-store elimination could therefore treat an overwriting store as inevitable even when another path entered a divergent region,
where a trap could expose the earlier memory state.

Reject cross-block dead-store candidates when the original store can reach a divergent block without first passing through the overwriter. Add a CLIF regression covering the divergent path.

Testing:

• all 38 alias-analysis filetests
• all 187 cranelift-codegen unit tests
• 22 doctests
• cargo clippy -p cranelift-codegen --all-targets -- -D warnings
• original WAST reproducer from Dead-store-elimintation removes visible stores from divergent paths #14053