Minor performance optimization

Co-authored-by: epsi1on <ehsan.ma@gmx.com>

Author: wo80

CSparse/Complex/SparseMatrix.cs

@@ -171,15 +171,18 @@ public override void Multiply(ReadOnlySpan<Complex> x, Span<Complex> y)
             }
 
             int end;
+            Complex xj;
 
             for (int j = 0; j < columns; j++)
             {
                 end = ap[j + 1];
 
+                xj = x[j];
+
                 // Loop over the rows
                 for (int k = ap[j]; k < end; k++)
                 {
-                    y[ai[k]] += x[j] * ax[k];
+                    y[ai[k]] += xj * ax[k];
                 }
             }
         }

CSparse/Double/SparseMatrix.cs

@@ -171,15 +171,18 @@ public override void Multiply(ReadOnlySpan<double> x, Span<double> y)
             }
 
             int end;
+            double xj;
 
             for (int j = 0; j < columns; j++)
             {
                 end = ap[j + 1];
 
+                xj = x[j];
+
                 // Loop over the rows.
                 for (int k = ap[j]; k < end; k++)
                 {
-                    y[ai[k]] += x[j] * ax[k];
+                    y[ai[k]] += xj * ax[k];
                 }
             }
         }