[msan] Improve packed multiply-add instrumentation (#152941) The current instrumentation has false positives: if there is a single uninitialized bit in any of the operands, the entire output is poisoned. This does not take into account that multiplying an uninitialized value with zero results in an initialized zero value. This step allows elements that are zero to clear the corresponding shadow during the multiplication step. The horizontal add step and accumulation step (if any) are modeled using bitwise OR. Future work can apply this improved handler to the AVX512 equivalent intrinsics (x86_avx512_pmaddw_d_512, x86_avx512_pmaddubs_w_512.) and AVX VNNI intrinsics.
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