Split helper functions

Split helper functions

This splits a couple of address-of and sizeof functions, so that the
body can be reused by the (coming) new expression code.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* eval.c (evaluate_subexp_for_address_base): New function.
	(evaluate_subexp_for_address): Use it.
	(evaluate_subexp_for_sizeof_base): New function.
	(evaluate_subexp_for_sizeof): Use it.