PR c++/7503
	* tree.c (lvalue_p_1):  Disallow MIN_EXPR and MAX_EXPR as lvalues
	if either operand has side-effects.
	* typeck.c (rationalize_conditional_expr): Assert that neither
	operand of MIN_EXPR or MAX_EXPR has side-effects.
	(build_modify_expr):  Add support for MIN_EXPR and MAX_EXPR.
	Check that the "lhs" is a valid lvalue, i.e. that neither operand
	of a MIN_EXPR or MAX_EXPR has a side-effect.

	* g++.dg/opt/pr7503-1.C: New testcase for COND_EXPR lvalues.
	* g++.dg/opt/pr7503-2.C: New testcase for <? and >? lvalues.
	* g++.dg/opt/pr7503-3.C: New testcase for invalid <? lvalue errors.
	* g++.dg/opt/pr7503-4.C: New testcase for <?= and >?= assignments.
	* g++.dg/opt/pr7503-5.C: New testcase for side-effects with <?=.

From-SVN: r87810