[MLIR][SPIRVToLLVM] Conversion of SPIR-V variable op

The patch introduces the conversion pattern for function-level
`spv.Variable`. It is modelled as `llvm.alloca` op. If initialized, then
additional store instruction is used. Note that there is no initialization
for arrays and structs since constants of these types are not supported in
LLVM dialect yet. Also, at the moment initialisation is only possible via
`spv.constant` (since `spv.GlobalVariable` conversion is not implemented
yet).

The input code has some scoping is not taken into account and will be
addressed in a different patch.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D84224