Change matMul expression to fix Windows inconsistency. (#1845)

BUG

Author: annxingyuan

src/backends/webgl/mulmat_packed_gpu.ts

@@ -25,8 +25,8 @@ export class MatMulPackedProgram implements GPGPUProgram {
 
   constructor(
       aShape: [number, number, number], outputShape: [number, number, number],
-      transposeA = false, transposeB = false,
-      addBias = false, activation: string = null) {
+      transposeA = false, transposeB = false, addBias = false,
+      activation: string = null) {
     this.outputShape = outputShape;
 
     const sharedDim = transposeA ? aShape[1] : aShape[2];
@@ -62,8 +62,10 @@ export class MatMulPackedProgram implements GPGPUProgram {
           vec4 a = getMatrixA(rc.x, ${aSample});
           vec4 b = getMatrixB(rc.x, ${bSample});
 
-          result += (${aSwizzle[0]} * ${bSwizzle[0]}) + (${aSwizzle[1]} * ${
-        bSwizzle[1]});
+          // These swizzled products need to be separately added.
+          // See: https://github.com/tensorflow/tfjs/issues/1735
+          result += (${aSwizzle[0]} * ${bSwizzle[0]});
+          result += (${aSwizzle[1]} * ${bSwizzle[1]});
         }
         return result;
       }