* fold-const.c (negate_expr_p): MULT_EXPRs and RDIV_EXPRs are easy

	* fold-const.c (negate_expr_p): MULT_EXPRs and RDIV_EXPRs are easy
	to negate if either operand is easy to negate, if we don't care
	about sign-dependent rounding.
	(negate_expr): Make the logic to negate a REAL_CST explicit.
	Attempt to negate a MULT_EXPR or RDIV_EXPR by negating an operand
	that's easy to negate, if we don't honor sign-dependent rounding.
	(fold <MULT_EXPR>): Optimize -A * B as A * -B if B is easy to
	negate, and the symmetric A * -B as -A * B if A is easy to negate.
	(fold <RDIV_EXPR>): Likewise, optimize -A/B and C/-D as A/-B and
	-C/D if B and C are cheap to negate.  Add an explicit rule to
	optimize X/-1.0 as -X when we don't care about signaling NaNs.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@70455 138bc75d-0d04-0410-961f-82ee72b054a4