LATX: remove overwritten intermediate 32-bit zero extensions

[luzeng87]

Summary

• identify self zero-extension sequences generated for 32-bit x86 results
• remove an intermediate extension when only low-32-bit read-modify-write operations follow and a later extension overwrites it
• stop at branches, labels, memory operations, faults, or any wide use of the register

Validation

• clean 3A6000 build: passed
• chained 32-bit arithmetic with a final 64-bit architectural-state check: passed
• JIT, cold AOT, and hot AOT: passed with a non-empty AOT file
• git diff --check: passed

[luzeng87]

This optimizer crosses LISA_X86_INST markers while temporarily leaving an architectural 32-bit result with stale upper bits. An asynchronous guest signal may materialize register state at such an instruction boundary. Without signal-aware state reconstruction or a proof that the transformation stays inside one guest instruction, this is a correctness risk. Closing it for redesign.