[spirv] Basic validity of SPV_ModuleOp

This CL adds SPV_ModuleEndOp for terminating the only block inside a
SPV_ModuleOp's only region. Verification now enforces a spv.module only
contains func or spv.* ops and no external or nested functions are
present. Because of the structural requirement of a block, spv.Return
is also added in this CL.

PiperOrigin-RevId: 251510706