[mlir][spirv] Support conversion of `CopySignOp` to spirv for 1D vector with 1 element

Conversion of CopySignOp to SPIRV is supported for scalar and vectors but not 1D vectors with 1 element (aka vector<1xf32>). This revisions adds supports this by treating them as scalars.

An alternative solution would be to allow 0D vectors for SPIRV, but the spec [0] strictly defines the vector type as non-0D.
"Vector: An ordered homogeneous collection of two or more scalars. Vector sizes are quite restrictive and dependent on the execution model."

[0] https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#_types

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D139518