Fix a miscompile in the DAG combiner. Previously, we would incorrectly

try to reduce the width of this load, and would end up transforming:

  (truncate (lshr (sextload i48 <ptr> as i64), 32) to i32)
to
  (truncate (zextload i32 <ptr+4> as i64) to i32)

We lost the sext attached to the load while building the narrower i32
load, and replaced it with a zext because lshr always zext's the
results. Instead, bail out of this combine when there is a conflict
between a sextload and a zext narrowing. The rest of the DAG combiner
still optimize the code down to the proper single instruction:

  movswl 6(...),%eax

Which is exactly what we wanted. Previously we read past the end *and*
missed the sign extension:

  movl 6(...), %eax

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169802 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -5068,11 +5068,15 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
       // At this point, we must have a load or else we can't do the transform.
       if (!isa<LoadSDNode>(N0)) return SDValue();
 
+      // Because a SRL must be assumed to *need* to zero-extend the high bits
+      // (as opposed to anyext the high bits), we can't combine the zextload
+      // lowering of SRL and an sextload.
+      if (cast<LoadSDNode>(N0)->getExtensionType() == ISD::SEXTLOAD)
+        return SDValue();
+
       // If the shift amount is larger than the input type then we're not
       // accessing any of the loaded bytes.  If the load was a zextload/extload
       // then the result of the shift+trunc is zero/undef (handled elsewhere).
-      // If the load was a sextload then the result is a splat of the sign bit
-      // of the extended byte.  This is not worth optimizing for.
       if (ShAmt >= cast<LoadSDNode>(N0)->getMemoryVT().getSizeInBits())
         return SDValue();
     }