### Description
This PR implements a minimal version of function signature conversion to
unroll vectors into 1D and with a size supported by SPIR-V (2, 3 or 4
depending on the original dimension). This PR also includes new unit
tests that only check for function signature conversion.
### Future Plans
- Check for capabilities that support vectors of size 8 or 16.
- Set up `OneToNTypeConversion` and `DialectConversion` to replace the
current implementation that uses `GreedyPatternRewriteDriver`.
- Introduce other vector unrolling patterns to cancel out the
`vector.insert_strided_slice` and `vector.extract_strided_slice` ops and
fully legalize the vector types in the function body.
- Handle `func::CallOp` and declarations.
- Restructure the code in `SPIRVConversion.cpp`.
- Create test passes for testing sets of patterns in isolation.
- Optimize the way original shape is splitted into target shapes, e.g.
`vector<5xi32>` can be splitted into `vector<4xi32>` and
`vector<1xi32>`.
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Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>
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