[webgpu] More preparation for element-wise binary op restructuring (#7666)

The current code isn't great in that the vec4 shaders have diverged from
the scalar ones more than necessary. Here is the common preparation
work, so that following refactoring can be done on a per-op basis.

Author: hujiajie

tfjs-backend-webgpu/src/binary_op_util.ts

@@ -41,15 +41,6 @@ export enum BinaryOpType {
   SUB
 }
 
-const CHECK_NAN_SNIPPET = `
-  resultTemp = select(resultTemp, valueForNaN, isNaN | isnan(a) | isnan(b));`;
-
-const CHECK_NAN_SNIPPET_VEC4 = `
-  resultTemp = select(
-      resultTemp, vec4<f32>(valueForNaN),
-      vec4<bool>(isNaN) | isnanVec4(a) | isnanVec4(b));
-  `;
-
 const ADD = 'return a + b;';
 const ATAN2 = 'var resultTemp = atan2(a, b);';
 // (Ar + Ai)(Br + Bi) =
@@ -183,9 +174,10 @@ const SUB = 'return a - b;';
 
 export function getBinaryOpString(
     type: BinaryOpType, useVec4?: boolean): string {
+  let doOpSnippet: string;
+
   // Ops with NaN check
   do {
-    let doOpSnippet: string;
     switch (type) {
       case BinaryOpType.ATAN2:
         doOpSnippet = ATAN2;
@@ -208,13 +200,34 @@ export function getBinaryOpString(
       default:
         continue;
     }
+
+    let isNaN: string;
+    let dTypeN: string;
+    let boolN: string;
+    if (useVec4) {
+      isNaN = 'isnanVec4';
+      dTypeN = 'vec4<f32>';
+      boolN = 'vec4<bool>';
+    } else {
+      isNaN = 'isnan';
+      dTypeN = 'f32';
+      boolN = 'bool';
+    }
+
     return `
+      let aIsNaN = ${isNaN}(a);
+      let aPostLegalization = select(a, ${dTypeN}(42), aIsNaN);
+      let bIsNaN = ${isNaN}(b);
+      let bPostLegalization = select(b, ${dTypeN}(42), bIsNaN);
       let isNaN = false;
       let valueForNaN = uniforms.NAN;
       {
+        let a = aPostLegalization;
+        let b = bPostLegalization;
         ${doOpSnippet}
-        ${useVec4 ? CHECK_NAN_SNIPPET_VEC4 : CHECK_NAN_SNIPPET}
-        return resultTemp;
+        return select(
+            resultTemp, ${dTypeN}(valueForNaN),
+            ${boolN}(isNaN) | aIsNaN | bIsNaN);
       }
     `;
   } while (false);
@@ -256,6 +269,10 @@ export function getBinaryOpString(
     case BinaryOpType.SUB:
       return SUB;
     default:
-      throw new Error(`BinaryType ${type} is not implemented!`);
+      // throw new Error(`BinaryType ${type} is not implemented!`);
   }
+  return `
+    ${doOpSnippet}
+    return resultTemp;
+  `;
 }