Ensure Hypot for double handles insignificant results

Author: tannergooding

src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/VectorMath.cs

@@ -336,6 +336,13 @@ public static TVectorDouble HypotDouble<TVectorDouble, TVectorUInt64>(TVectorDou
 
             TVectorUInt64 expDiff = xExp - yExp;
 
+            // Cover cases where x or y is insignifican compared to the other
+            TVectorDouble insignificanMask = Unsafe.BitCast<TVectorUInt64, TVectorDouble>(
+                TVectorUInt64.GreaterThanOrEqual(expDiff, TVectorUInt64.Create(SignificandLength + 1)) &
+                TVectorUInt64.LessThanOrEqual(expDiff, TVectorUInt64.Create(unchecked((ulong)(-SignificandLength - 1))))
+            );
+            TVectorDouble insignificantResult = ax + ay;
+
             // To prevent overflow, scale down by 2^+600
             TVectorUInt64 expBiasP500 = TVectorUInt64.Create(double.ExponentBias + 500);
             TVectorUInt64 scaleDownMask = TVectorUInt64.GreaterThan(xExp, expBiasP500) | TVectorUInt64.GreaterThan(yExp, expBiasP500);
@@ -427,6 +434,7 @@ public static TVectorDouble HypotDouble<TVectorDouble, TVectorUInt64>(TVectorDou
             // the inputs is NaN. Otherwise if either input
             // is NaN, we return NaN
 
+            result = TVectorDouble.ConditionalSelect(insignificanMask, insignificantResult, result);
             result = TVectorDouble.ConditionalSelect(nanMask, TVectorDouble.Create(double.NaN), result);
             result = TVectorDouble.ConditionalSelect(infinityMask, TVectorDouble.Create(double.PositiveInfinity), result);