LATX: remove redundant IR2 constants and address operations

[luzeng87]

Summary

• track LASX immediate values across adjacent guest instructions and omit repeated XVLDI operations
• invalidate tracked values on register writes, labels, branches, and indirect jumps
• split low signed 12-bit RIP-relative displacements from JIT address materialization
• use the low part directly as the host load/store displacement
• preserve the existing relocation form during AOT pre-translation

Validation

• clean 3A6000 build: passed
• AVX conversion, packing, multiply, shuffle, extract, insert, and broadcast fixtures: passed
• JIT, cold AOT, and hot AOT modes: passed with non-empty AOT files
• DCO sign-offs present on all commits

This PR supersedes the separate RIP-relative displacement PR. It remains a draft while grouped checks run.

[luzeng87]

Superseded by the consolidated #438 draft. The LASX immediate and RIP-relative address changes, with focused JIT/AOT coverage, are retained there.