wgsl: Convert quantizeToF16 to used hfround (#3118)

Instead of passing the input through a F16Array, use the library
provided function hfround. hfround is a fast look up table based
rounding function for f16.

Benchmarking locally this provides a ~20% improvement to fma interval
calculations, which are particularly sensitive to quantization
cost. Overall I was seeing more on the order of ~10% improvement.

src/webgpu/util/math.ts

@@ -3,6 +3,7 @@ import { assert } from '../../common/util/util.js';
 import {
   Float16Array,
   getFloat16,
+  hfround,
   setFloat16,
 } from '../../external/petamoriken/float16/float16.js';
 
@@ -2021,13 +2022,9 @@ export function quantizeToF32(num: number): number {
   return quantizeToF32Data[0];
 }
 
-/** Statically allocate working data, so it doesn't need per-call creation */
-const quantizeToF16Data = new Float16Array(new ArrayBuffer(2));
-
 /** @returns the closest 16-bit floating point value to the input */
 export function quantizeToF16(num: number): number {
-  quantizeToF16Data[0] = num;
-  return quantizeToF16Data[0];
+  return hfround(num);
 }
 
 /** Statically allocate working data, so it doesn't need per-call creation */