c++: Partially revert patch for PR66139.

c++: Partially revert patch for PR66139.

The patch for 66139 exposed a long-standing bug with
split_nonconstant_init (since 4.7, apparently): initializion of individual
elements of an aggregate are not a full-expressions, but
split_nonconstant_init was making full-expressions out of them.  My fix for
66139 extended the use of split_nonconstant_init, and thus the bug, to
aggregate initialization of temporaries within an expression, in which
context (PR94041) the bug is more noticeable.  PR93922 is a problem with my
implementation strategy of splitting out at gimplification time, introducing
function calls that weren't in the GENERIC.  So I'm going to revert the
patch now and try again for GCC 11.

gcc/cp/ChangeLog
2020-03-10  Jason Merrill  <jason@redhat.com>

	PR c++/93922
	PR c++/94041
	PR c++/52320
	PR c++/66139
	* cp-gimplify.c (cp_gimplify_init_expr): Partially revert patch for
	66139: Don't split_nonconstant_init.  Remove pre_p parameter.