Treat union fields as aliasing each other in value_analysis

[ojuschugh1]

Fixes #161898

The problem was that for_each_aliasing_place only considered enum variants and the discriminant as aliasing, so writing to one union field left the state of the other fields alone. JumpThreading then threaded the second switchInt on u.a using the value from before u.b = 5, and DataflowConstProp folded the read of u.a to a stale constant in the same way. Union field places have only been in the map since #127087 began collecting places syntactically; before that iter_fields skipped unions.

I made for_each_aliasing_place flood everything under a union as soon as the walk reaches it, since all non-ZST union fields start at offset 0 and overlap. The alternative of not registering places below unions would also work but loses threading on union reads with no write in between.

Added a mir-opt test for each pass.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @dingxiangfei2009 (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Please see the contribution instructions and our LLM policy for more information.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 77 candidates
• Random selection from 19 candidates

[ojuschugh1]

Hey @TaKO8Ki @dingxiangfei2009 could you kindly review this PR? Thanks