[stdlib] Fix sign extension in word -> multi-word integer conversions

This fixes integer conversion issues on 32-bit platforms.

Author: lorentey

stdlib/public/core/Integers.swift.gyb

@@ -2294,7 +2294,7 @@ ${unsafeOperationComment(x.operator)}
 
   @inline(__always)
   public init<T : BinaryInteger>(extendingOrTruncating source: T) {
-    if Self.bitWidth <= ${word_bits} || source.bitWidth <= ${word_bits} {
+    if Self.bitWidth <= ${word_bits} {
       self = Self.init(_truncatingBits: source._lowWord)
     }
     else {

test/stdlib/Integers.swift.gyb

@@ -738,6 +738,18 @@ dwTests.test("inits") {
   _ = DWU16(UInt32.max)
 }
 
+dwTests.test("TwoWords") {
+  typealias DW = DoubleWidth<Int>
+
+  expectEqual(-1 as DW, DW(extendingOrTruncating: -1 as Int8))
+
+  expectNil(Int(exactly: DW(Int.min) - 1))
+  expectNil(Int(exactly: DW(Int.max) + 1))
+
+  expectTrue(DW(Int.min) - 1 < Int.min)
+  expectTrue(DW(Int.max) + 1 > Int.max)
+}
+
 dwTests.test("Bitshifts") {
   typealias DWU16 = DoubleWidth<UInt8>
   typealias DWU32 = DoubleWidth<DWU16>