[mlir][spirv] Lower allocation/deallocations of workgroup memory.

This allocation of a workgroup memory is lowered to a
spv.globalVariable. Only static size allocation with element type
being int or float is handled. The lowering does account for the
element type that are not supported in the lowered spv.module based on
the extensions/capabilities and adjusts the number of elements to get
the same byte length.

Differential Revision: https://reviews.llvm.org/D80411