Preserve required SSA evaluations

[1sgtpepper]

Motivation

Fixes #29563.

This PR updates SSA constant propagation to track definitions whose evaluation must remain live even when their resulting value can no longer affect the final value.

The pass now carries required-evaluation roots through SSA paths and checks those roots before applying rewrites that drop an expression, including zero absorption, boolean absorption, self-comparison folding, ternary folding, and composite field forwarding.

Test Plan

Added regressions covering:

• checked subtraction preserved through zero absorption;
• required evaluation preserved through aliases and ternaries;
• composite field forwarding only when it preserves required evaluation;
• execution behavior for checked subtraction underflow.

Validated with:

• cargo +nightly-x86_64-pc-windows-gnu fmt --check
• cargo +1.96.0-x86_64-pc-windows-gnu clippy -p leo-passes --all-targets --locked -- -D warnings
• focused SSA const-prop pass tests
• focused ssa_checked_sub execution test

Related PRs

N/A