cast_possible_truncation: don't lint when usize is masked to fit target type

[Souradip121]

Fixes the false positive in cast_possible_truncation where masking a usize value to fit within a u32 (or smaller) before casting still triggered a warning.

Closes #16854

What was wrong

// Before (incorrectly warned):
let remaining = (levels.len().saturating_sub(n) & u32::MAX as usize) as u32;
let remaining = (levels.len().saturating_sub(n) & 0xFFFF_FFFF_usize) as u32;

// After (no warning — value is provably within u32 range):
let remaining = (levels.len().saturating_sub(n) & u32::MAX as usize) as u32;
let remaining = (levels.len().saturating_sub(n) & 0xFFFF_FFFF_usize) as u32;

// Unmasked cast still warns correctly:
let remaining = levels.len() as u32;
//              ^^^^^^^^^^^^^^^^^ cast_possible_truncation

Root cause (two bugs)

1. get_constant_bits couldn't evaluate u32::MAX as usize — ConstEvalCtxt has no ExprKind::Cast handler, so it returned None for cast expressions even when the inner value is a known constant. Fixed by peeling through cast layers to reach the underlying constant.

2. apply_reductions result was ignored for usize → fixed-size int casts — the (true, false) match arm checked to_nbits <= 32 unconditionally, never consulting the reduced from_nbits. Fixed by adding && from_nbits > to_nbits.

changelog: [cast_possible_truncation]: no longer lints when a usize value is masked to fit the target integer type before casting

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[github-actions]

Lintcheck changes for 6a8bc47

Lint	Added	Removed	Changed
clippy::cast_possible_truncation	0	5	893

This comment will be updated if you push new changes

[simonsso]

Could also suggest user to explicitly mask bits if that is what they intended to do.
let _ = y & 0xFFFF_FFFF as u32

View changes since the review

[Souradip121]

@simonsso good call, pushed 6a8bc47 which adds the mask alternative. The lint now offers a second help: alongside the existing try_from suggestion. Scoped narrowly to unsigned to unsigned integer casts where masking has clean semantics, so signed types, enum casts, and inferred _ targets fall back to try_from only.