Backport fix for bitmatrix matmul regression 1.11.1 to 1.11.2 (#57387)

Fix #56954.

Author: jakobnissen

stdlib/LinearAlgebra/src/matmul.jl

@@ -892,6 +892,7 @@ Base.@constprop :aggressive generic_matmatmul!(C::AbstractVecOrMat, tA, tB, A::A
         @inbounds for n in BxN, k in BxK
             # Balpha = B[k,n] * alpha, but we skip the multiplication in case isone(alpha)
             Balpha = _rmul_alpha(B[k,n])
+            !ismissing(Balpha) && iszero(Balpha) && continue
             @simd for m in AxM
                 C[m,n] = muladd(A[m,k], Balpha, C[m,n])
             end

stdlib/LinearAlgebra/test/matmul.jl

@@ -755,6 +755,7 @@ import LinearAlgebra: Adjoint, Transpose
 (*)(x::RootInt, y::Integer) = x.i * y
 adjoint(x::RootInt) = x
 transpose(x::RootInt) = x
+Base.zero(::RootInt) = RootInt(0)
 
 @test Base.promote_op(*, RootInt, RootInt) === Int