Make .significandWidth do the right thing for exact powers of two

Resolves SR-2868.

Author: hooman

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -668,6 +668,7 @@ extension ${Self}: BinaryFloatingPoint {
   public var significandWidth: Int {
     let trailingZeros = significandBitPattern.countTrailingZeros
     if isNormal {
+      guard significandBitPattern != 0 else { return 0 }
       return ${Self}.significandBitCount - trailingZeros
     }
     if isSubnormal {

test/stdlib/FloatingPoint.swift.gyb

@@ -395,6 +395,18 @@ FloatingPoint.test("${Self}/LosslessStringConvertible") {
   checkLosslessStringConvertible(instances)
   expectTrue(Float(String(Float.nan))!.isNaN)
 }
+
+FloatingPoint.test("${Self}.significandWidth") {
+  expectEqual(-1, ${Self}(0).significandWidth)
+  expectEqual(-1, ${Self}.infinity.significandWidth)
+  expectEqual(-1, ${Self}.nan.significandWidth)
+  expectEqual(${Self}.significandBitCount, ${Self}(1).nextUp.significandWidth)
+  expectEqual(${Self}.significandBitCount, ${Self}(1).nextDown.significandWidth)
+  expectEqual(0, ${Self}.ulpOfOne.significandWidth)
+  expectEqual(0, ${Self}(1).significandWidth)
+  expectEqual(0, ${Self}(2).significandWidth)
+  expectEqual(1, ${Self}(3).significandWidth)
+}
 % if Self == 'Float80':
 #endif
 % end