[mlir][spirv] Add lowering for load/store zero-rank memref from std to SPIR-V.

[hanchung]

This might be wrong. Do we expect to load/store f/i32 instead of spv.array<1 x f/i32 [4]>?

[mravishankar]

Might be worth checking that the serialization/deserialization works as well

mlir/test/Conversion/StandardToSPIRV/std-to-spirv.mlir

319

Would be good to check the indices here to make sure it is doing spv.AccessChain %arg0[0]

[hanchung]

Load/Store the element type directly, and check the indices for AccessChain.

[mravishankar]

This looks fine to me. Thanks for adding this.
But please add serialization tests as well

[hanchung]

Add serialization tests.