tree: Change error_operand_p to an inline function

tree: Change error_operand_p to an inline function

I've thought for a while that many of the macros in tree.h and such should
become inline functions.  This one in particular was confusing Coverity; the
null check in the macro made it think that all code guarded by
error_operand_p would also need null checks.

gcc/ChangeLog:

	* tree.h (error_operand_p): Change to inline function.