spirv-opt: missed optimization opportunity for VectorShuffle over CompositeConstruct

The following pattern is currently not optimized by spirv-opt with either `-O` or `-Os`

```
%10 =
%11 = OpCompositeExtract %int %10 0
%12 = OpCompositeExtract %int %10 1
%13 = OpCompositeConstruct %v3int %11 %12 %int_0
%14 = OpVectorShuffle %v2int %13 %13 0 1
```

I think this should be simplified to:

```
%10 =
%14 = %10
```

Start in the spirv-opt optimization paths exercised by -O and -Os, and locate handling for VectorShuffle together with CompositeConstruct. Reproduce the shown instruction pattern, then verify that both optimization modes reduce it to the original vector; add or run a regression test covering the transformation.

Written by the indexing model from the issue text.