fix(vpto): lower signless i8*i8->i32 MAD to llvm.hivm.MAD.s8.c310

[Zhendong404]

Summary

• cherry-pick PR fix(vpto): support signless i8*i8->i32 mad lowering (https://github.com/mouliangyu/PTOAS/issues/381) #389's signless i8*i8->i32 MAD lowering support onto feature-vpto-backend
• fix the integer MAD LLVM callee to use llvm.hivm.MAD.s8.c310
• add a VPTO cube lit regression that checks the i8 -> i32 path emits the correct intrinsic

Validation

• cmake --build build --target ptoas -j $(nproc)
• build/tools/ptoas/ptoas --pto-arch=a5 --pto-backend=vpto test/lit/vpto/cube/mad_semantic_vpto_llvm.pto -o /tmp/mad_semantic_vpto_llvm.o --mlir-print-ir-after=convert-func-to-llvm 2>&1 | rg -n "llvm\\.hivm\\.MAD\\.(f322f32\\.c310|s8\\.c310|s82s32\\.c310)|mad_semantic_vpto_llvm_i8"
• mkdir -p /tmp/ptoas-lit && ( build/tools/ptoas/ptoas --pto-arch=a5 --pto-backend=vpto test/lit/vpto/cube/mad_semantic_vpto_llvm.pto -o /tmp/ptoas-lit/mad.o --mlir-print-ir-after=convert-func-to-llvm 2>&1 || true ) | /home/zhangzhendong/ptoas-workspace/llvm-project/build-shared/bin/FileCheck test/lit/vpto/cube/mad_semantic_vpto_llvm.pto

Fixes #381
Supersedes #389