TSL: Fix assignment to swizzled for var intent

[sunag]

Fixes #31638

Description

Fix assignment to swizzled for var intent.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.9 79.1	338.9 79.1	+0 B +0 B
WebGPU	575.21 158.73	575.27 158.73	+54 B +3 B
WebGPU Nodes	573.82 158.49	573.88 158.49	+54 B +3 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	470.64 113.87	470.64 113.87	+0 B +0 B
WebGPU	646.28 174.8	646.34 174.8	+54 B +2 B
WebGPU Nodes	600.46 163.98	600.51 163.98	+54 B +2 B