* fold-const.c (negate_expr_p, negate_expr): Optimize -(A+B) into
	either (-A)-B or (-B)-A, if A or B is easily negated respectively.
	(fold) <MINUS_EXPR>: Optimize (A*C) - (B*C) -> (A-B)*C for both
	integer types and floating point with unsafe_math_optimizations.
	Add similar optimization for (A*C1) - (A*C2) -> A*(C1-C2).
	Optimize A - B as A + (-B), if B is easily negated.

From-SVN: r77453