[MLIR][SPIRVToLLVM] Conversion of load and store SPIR-V ops

This patch introduces conversion pattern for `spv.Store` and `spv.Load`.
Only op with `Function` Storage Class is supported at the moment
because `spv.GlobalVariable` has not been introduced yet. If the op
has memory access attribute, then there are the following cases.
If the access is `Aligned`, add alignment to the op builder. Otherwise
the conversion fails as other cases are not supported yet because they
require additional attributes for `llvm.store`/`llvm.load` ops: e.g.
`volatile` and `nontemporal`.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D84236