Add tests for swizzle assignment language feature

[sudonatalie]

Tests for swizzle assignment should include:

• simple variables with a variety of single and multi-element swizzle assignments on different types and swizzle names
• swizzle assignments on other valid reference types / address spaces than simple function vars
• chained swizzles
• pointer to swizzles
• compound assignments (i.e. v.xy += 1)
• invalid assignments (i.e. v.xx = ..., out of bounds, etc.)