LATX: fold RIP-relative displacement into JIT loads

[luzeng87]

Summary

• split the low signed 12 bits from a RIP-relative guest address during JIT translation
• keep the aligned base in the address materialization sequence and return the low part as the host load/store displacement
• retain the original relocation form during AOT pre-translation

Validation

• clean 3A6000 build: passed
• AVX conversion, packing, rounded multiply, shuffle, extract/insert, and broadcast instruction tests: passed
• JIT, cold AOT, and hot AOT: passed with non-empty AOT files
• git diff --check: passed

[luzeng87]

Superseded by grouped IR2 and JIT simplification PR #452. The RIP-relative displacement change remains a separate commit there.