Make SPIR-V spv.EntryPoint and spv.ExecutionMode consistent with SPIR-V spec

This CL changes the Op definition of spirv::EntryPointOp and
spirv::ExecutionModeOp to be consistent with the SPIR-V spec.
1) The EntryPointOp doesn't return a value
2) The ExecutionModeOp takes as argument, the SymbolRefAttr to refer
to the function, instead of the result of the EntryPointOp.

Following this, the spirv::EntryPointType is no longer necessary, and
is removed.

PiperOrigin-RevId: 258964027