Fix parsing/printing of spv.globalVariable and spv._address_of

Fix parsing/printing of spv.globalVariable and  spv._address_of

Change the prining/parsing of spv.globalVariable to print the type of
the variable after the ':' to be consistent with MLIR convention.
The spv._address_of should print the variable type after the ':'. It was
mistakenly printing the address of the return value. Add a (missing)
test that should have caught that.
Also move spv.globalVariable and spv._address_of tests to
structure-ops.mlir.

PiperOrigin-RevId: 264204686