[mlir][amx] Simplify intrinsic generation (#140559)
Replaces separate amx named intrinsic operations with direct calls to
LLVM intrinsic functions.
The existing amx tests are updated and expanded.
The separate conversion step translating amx intrinsics into LLVM IR is
eliminated. Instead, this step is now performed by the existing llvm
dialect infrastructure.
Related RFC:
https://discourse.llvm.org/t/rfc-simplify-x86-intrinsic-generation/85581/7
