Add fast path in generic matmul

This manually adds the critical optimisation investigated in Julia issue
56954. While we could rely on LLVM to continue doing this optimisation,
it's more robust to add it manually.

Author: jakobnissen

src/matmul.jl

@@ -1021,6 +1021,7 @@ function _generic_matmatmul_nonadjtrans!(C, A, B, alpha, beta)
     @inbounds for n in axes(B, 2), k in axes(B, 1)
         # Balpha = B[k,n] * alpha, but we skip the multiplication in case isone(alpha)
         Balpha = @stable_muladdmul MulAddMul(alpha, false)(B[k,n])
+        iszero(Balpha) && continue
         @simd for m in axes(A, 1)
             C[m,n] = muladd(A[m,k], Balpha, C[m,n])
         end