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

This fixes integer conversion issues on 32-bit platforms.

(cherry picked from commit 735fe97)

Author: lorentey

stdlib/public/core/Integers.swift.gyb

@@ -2260,7 +2260,7 @@ ${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

@@ -622,21 +622,4 @@ tests.test("signum/concrete") {
 }
 
 
-dwTests.test("Words") {
-  expectEqualSequence((0 as DoubleWidth<Int8>).words, [0])
-  expectEqualSequence((1 as DoubleWidth<Int8>).words, [1])
-  expectEqualSequence((-1 as DoubleWidth<Int8>).words, [UInt.max])
-  expectEqualSequence((256 as DoubleWidth<Int8>).words, [256])
-  expectEqualSequence((-256 as DoubleWidth<Int8>).words, [UInt.max - 255])
-  expectEqualSequence(DoubleWidth<Int8>.max.words, [32767])
-  expectEqualSequence(DoubleWidth<Int8>.min.words, [UInt.max - 32767])
-
-  expectEqualSequence((0 as Int1024).words, 
-    repeatElement(0 as UInt, count: 1024 / UInt.bitWidth))
-  expectEqualSequence((-1 as Int1024).words,
-    repeatElement(UInt.max, count: 1024 / UInt.bitWidth))
-  expectEqualSequence((1 as Int1024).words, 
-      [1] + Array(repeating: 0, count: 1024 / UInt.bitWidth - 1))
-}
-
 runAllTests()