Split value_fetch_lazy

While reading value_fetch_lazy, I thought it would be good to split it
in small functions (especially the part that handles lval_register).

gdb/ChangeLog:

	* value.c (value_fetch_lazy_bitfield): New.
	(value_fetch_lazy_memory): New.
	(value_fetch_lazy_register): New.
	(value_fetch_lazy): Factor out to smaller functions.