Preserve signed zero on roundtrip

This commit removes the previous mechanism of differentiating
between "Debug" and "Display" formattings for the sign of -0 so as
to comply with the IEEE 754 standard's requirements on external
character sequences preserving various attributes of a floating
point representation.

In addition, numerous tests are fixed.

src/test/mir-opt/funky_arms.float_to_exponential_common.ConstProp.diff

@@ -51,7 +51,7 @@
       }
   
       bb2: {
-          discriminant(_6) = 2;            // scope 1 at $DIR/funky_arms.rs:21:17: 21:41
+          discriminant(_6) = 1;            // scope 1 at $DIR/funky_arms.rs:21:17: 21:41
           goto -> bb4;                     // scope 1 at $DIR/funky_arms.rs:19:16: 22:6
       }