[Codegen][CPU] Fill in the bf16 and i8 ukernel bodies + e2e tests.

[bjacob]

Replaces the bf16 and i8-VNNI seeds' stub bodies with real SIMD implementations, both generic over the intrinsics_{m,n,k} unrolling factors and structured like the AMDGPU C ukernel they were adapted from (iree_uk_amdgpu_multi_mma_mfma_i32_16x16x32_i8): accumulators in registers, an outer loop over the K tiles (k_outer), and inside it the (intrinsics_m, intrinsics_n, intrinsics_k) unroll. The intrinsics_* arrive as constants at the inlined call site, so the loops fully unroll and the acc_regs arrays become fixed register files -- the bitcode-LTO equivalent of a C++ template, as the README describes.

• bf16 (MMA_X86_AVX512BF16_1x16x2_F32_BF16): one _mm512_dpbf16_ps per (m, n, k), with the LHS K-pair broadcast via set1_ps.
• i8 (MMA_X86_AVX512VNNI_16x16x2_I32_I8_CASTI16): the 16x16x2 tile is bit-compatible with the codegen path lowerX86Avx512Vnni16x16x2I8 -- one vpmovsxbw widen of each i8 panel to i16, the vpshufd / vbroadcasti32x4 fan-out, and 16 vpdpwssd over the block-interleaved (rlo, chi, rhi, clo) ACC layout. The i8 ukernel needs -mavx512bw for the widen, so it is added to the VNNI copts.

LLVMCPUSelectUKernels now only selects a ukernel when its bitcode actually exists (via attachUKernelBitcodeOnOp's bool return), so an MMAIntrinsic the cost model picks but for which no seed exists -- e.g. the M<->N-swapped MMA_X86_AVX512BF16_16x1x2_F32_BF16 -- falls back to codegen instead of dangling an undefined symbol.

Adds two execution/numerical tests, the first of the new C-bitcode ukernel path: e2e_matmul_cpu_dt_inner_tiled_llvm_ukernel_bf16_f32 (avx512bf16) and ..._i8_i32 (avx512vnni). Each compiles a data-tiled matmul with --iree-llvmcpu-enable-llvm-ukernels=inner_tiled, links the ukernel bitcode, runs on host and checks results against a reference -- exercising the operand threading and generic intrinsics_{m,n,k} unrolling that the IR-level lit tests cannot. Both were confirmed to actually select their ukernel (not silently fall back to codegen).

Progress towards #24574.