LATX, AVX: use direct widening multiply-add sequences

[luzeng87]

Summary

• lower VPMADDWD to widening even/odd multiplies plus addition without clearing and copying a temporary vector
• lower VPMADDUBSW with the mixed unsigned-byte/signed-byte widening multiply instructions
• preserve source/destination overlap by selecting a temporary sequence only when required
• apply the same lowering to LSX lane translation and native LASX translation

Validation

• clean 3A6000 build: passed
• VPMADDWD and VPMADDUBSW over 100,000 deterministic input sets: passed
• signed saturation and source-value checks: passed
• JIT, cold AOT, and hot AOT: passed with a non-empty AOT file
• git diff --check: passed

[luzeng87]

Superseded by grouped AVX lowering PR #448. The original commit remains separate inside that PR for review.