middle-end: Simplify subtract where both arguments are being bitwise inverted.

This adds a match.pd rule that drops the bitwwise nots when both arguments to a
subtract is inverted. i.e. for:

float g(float a, float b)
{
  return ~(int)a - ~(int)b;
}

we instead generate

float g(float a, float b)
{
  return (int)b - (int)a;
}

We already do a limited version of this from the fold_binary fold functions but
this makes a more general version in match.pd that applies more often.

gcc/ChangeLog:

	* match.pd: New bit_not rule.

gcc/testsuite/ChangeLog:

	* gcc.dg/subnot.c: New test.

gcc/match.pd

@@ -1308,6 +1308,11 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 (simplify
  (bit_not (plus:c (bit_not @0) @1))
  (minus @0 @1))
+/* (~X - ~Y) -> Y - X.  */
+(simplify
+ (minus (bit_not @0) (bit_not @1))
+  (with { tree utype = unsigned_type_for (type); }
+   (convert (minus (convert:utype @1) (convert:utype @0)))))
 
 /* ~(X - Y) -> ~X + Y.  */
 (simplify

gcc/testsuite/gcc.dg/subnot.c

@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-optimized" } */
+
+float g(float a, float b)
+{
+  return ~(int)a - ~(int)b;
+}
+
+/* { dg-final { scan-tree-dump-not "~" "optimized" } } */