[SPIRV] Support selection of G_CONCAT_VECTORS (#201686)
Implement the G_CONCAT_VECTOR opcode using `OpCompositeConstruct`. The
semantics are similar so the implementation is straightforward.
This opcode being generated is somewhat rare, in this case it seems to
have remained due to the non-power of 2 vector length ABI.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
