[mlir][gpu][spirv] Convert memref<mma_matrix> to spv.array<coopmatrix> (#212806)

In a gpu.func, allow allocating local arrays of gpu.mma_matrix, e.g.
`memref.alloca() : memref<Nx!gpu.mma_matrix<HxW, ...>`. Extend the SPIRV
conversion to convert such memrefs to `%ARR = spirv.Variable:
spv.ptr<spv.array<N x coopmatrix>, Function>`, which can be conveniently
indexed with `spirv.AccessChain %ARR[i]`. This enables arrays of CoopMMA
matrices both at the `gpu` and `spirv` levels.

Signed-off-by: Fabrizio Indirli <fabrizio.indirli@arm.com>
GitOrigin-RevId: d46f1e166a5692b816dfe373babc7ad760912711