Specialized mul for 3 diagonal arguments

[kshyatt]

Currently this will trigger a scalar indexing error because it eventually hits __muldiag in LinearAlgebra.jl.

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diff --git a/src/host/linalg.jl b/src/host/linalg.jl
index 2282528..055a6ac 100644
--- a/src/host/linalg.jl
+++ b/src/host/linalg.jl
@@ -244,9 +244,11 @@ function Base.:\(D::Diagonal{<:Any, <:AbstractGPUArray}, B::AbstractGPUVecOrMat)
     end
 end
 
-function LinearAlgebra.mul!(C::Diagonal{<:Any, <:AbstractGPUArray},
-                            A::Diagonal{<:Any, <:AbstractGPUArray},
-                            B::Diagonal{<:Any, <:AbstractGPUArray})
+function LinearAlgebra.mul!(
+        C::Diagonal{<:Any, <:AbstractGPUArray},
+        A::Diagonal{<:Any, <:AbstractGPUArray},
+        B::Diagonal{<:Any, <:AbstractGPUArray}
+    )
     dc = C.diag
     da = A.diag
     db = B.diag
diff --git a/test/testsuite/linalg.jl b/test/testsuite/linalg.jl
index 5770c85..9bfca0b 100644
--- a/test/testsuite/linalg.jl
+++ b/test/testsuite/linalg.jl
@@ -249,7 +249,7 @@
                 B = Diagonal(b)
                 A = Diagonal(a)
                 mul!(C, A, B)
-                @test collect(C.diag) ≈ collect(A.diag) .* collect(B.diag) 
+                @test collect(C.diag) ≈ collect(A.diag) .* collect(B.diag)
             end
         end