LATX, AVX: remove redundant VPSHUFB index masking

[luzeng87]

Summary

• pass the original VPSHUFB control vector directly to XVSHUF.B because ignored control bits do not affect lane selection
• retain explicit sign-bit zeroing for dynamic controls
• omit the sign-mask path when an immutable RIP-relative control table contains no zeroing entries

Validation

• clean 3A6000 build: passed
• lane selection and sign-bit zeroing over 100,000 deterministic input sets: passed
• JIT, cold AOT, and hot AOT: passed with a non-empty AOT file
• git diff --check: passed

[luzeng87]

A standalone VPSHUFB property test covering 100,000 source/control combinations fails in JIT as soon as the index mask is removed. The read-only-memory specialization is also unsafe because a translated block can outlive the sampled memory contents. Closing this optimization; neither path is correct as submitted.

[luzeng87]

Closed after a standalone VPSHUFB property test found incorrect control-byte handling. It is not included in any grouped PR.