Add regression test

Author: Manishearth

tests/ui/excessive_precision.fixed

@@ -60,4 +60,7 @@ fn main() {
 
     // issue #2840
     let num = 0.000_000_000_01e-10f64;
+
+    // issue #7744
+    let _ = 2.225_073_858_507_201e-_308_f64;
 }

tests/ui/excessive_precision.rs

@@ -60,4 +60,7 @@ fn main() {
 
     // issue #2840
     let num = 0.000_000_000_01e-10f64;
+
+    // issue #7744
+    let _ = 2.225_073_858_507_201_1e-308_f64;
 }

tests/ui/excessive_precision.stderr

@@ -78,5 +78,11 @@ error: float has excessive precision
 LL |     let bad_bige32: f32 = 1.123_456_788_888E-10;
    |                           ^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `1.123_456_8E-10`
 
-error: aborting due to 13 previous errors
+error: float has excessive precision
+  --> $DIR/excessive_precision.rs:65:13
+   |
+LL |     let _ = 2.225_073_858_507_201_1e-308_f64;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `2.225_073_858_507_201e-_308_f64`
+
+error: aborting due to 14 previous errors