pVectorExtractDynamic with spec constant index is not correct

I am running into #1178 when targeting metal:
- `half4 foo; foo[specialization constant]` ends up `foo[0]` in metal shader.
- Putting specialization constant first into a temp variable works around the issue, but only if optimizations are not used.
- A silly switch case successfully works around the issue even when optimizations are used.

I will try to fill in more details later.

Start with issue #1178 and the reported Metal shader output for pVectorExtractDynamic using a specialization-constant index. Compare the direct indexing case with the temporary-variable and switch-case workarounds, both with and without optimization. Done means the specialization-constant index is preserved correctly in optimized and unoptimized Metal output.

Written by the indexing model from the issue text.