This unifies the identity operation, which is used by SSA_NAME, 
PAREN_EXPR, OBJ_TYPE_REF and REAL_CST.

REAL_CST is using it incorrectly, but preserves current functionality.  
There will not be an SSA_NAME in the op1 position, so there is no point 
in having an op1_range routine.  That will be corrected in the next patch.

Bootstrap on x86_64-pc-linux-gnu and pass all regressions. Pushed.

Andrew