Hello,

this copies some more transformations from fold_negate_expr to match.pd. 
Mostly, I wanted to add (negate (pointer_diff @0 @1)), and couldn't find 
the corresponding transformation with minus_expr... We can see about 
generalizing a bit with conversions later, I wanted to have something at 
least.

Bootstrap+regtest on powerpc64le-unknown-linux-gnu.

gcc/ChangeLog:

2017-11-04  Marc Glisse  <marc.glisse@inria.fr>

 	* fold-const.c (negate_expr_p) [PLUS_EXPR, MINUS_EXPR]: Handle
 	non-scalar integral types.
 	* match.pd (negate_expr_p): Handle MINUS_EXPR.
 	(-(A-B), -(~A)): New transformations.

gcc/testsuite/ChangeLog:

2017-11-04  Marc Glisse  <marc.glisse@inria.fr>

 	* gcc.dg/tree-ssa/negminus.c: New test.

-- 
Marc Glisse