spirv: fix SpvOpSpecConstantOp with SpvOpVectorShuffle working with double-based...

spirv: fix SpvOpSpecConstantOp with SpvOpVectorShuffle working with double-based vecs

We need to pick two 32-bit values per component to perform the right shuffle operation.

v2 (Jason):
- Add assert to check matching bit sizes (Jason)
- Simplify the code to pick components (Jason)

v3:
- Switch on bit_size once (Jason)
- Add comment to explain the constant value for unused components (Erik)

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>