Lower checked integer shifts through the Whale adapter #863

Motivation

The initial Whale adapter rejects shifts. Wave already defines result width, signed right shift and invalid-count behavior; the adapter must preserve those decisions. Related to #93.

Scope

Use HIR types and check the original RHS before narrowing. Negative or out-of-range runtime counts trap; invalid evaluated constants remain frontend errors. Support the existing 8–128-bit runtime subset and full-value constant count checks. No native execution claim.

Implementation status: Already implemented in open PR #860. This issue restores missing tracking; do not duplicate the implementation. Review and final PR validation remain with that PR.

Completion criteria

• Preserve LHS result width and signed/unsigned right-shift semantics.
• Test original-count guards before narrowing across supported widths.
• Keep short-circuit behavior and reject wider unsupported runtime counts.