Fix constant used for pshufb mask when lowering v16i8 shuffles. Bug introduced in r157043. Fixes PR12908.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157236 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 4e4cf27..50a2d78 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -5805,7 +5805,7 @@
     for (unsigned i = 0; i != 16; ++i) {
       int EltIdx = MaskVals[i];
       EltIdx = (EltIdx < 16) ? 0x80 : EltIdx - 16;
-      pshufbMask.push_back(DAG.getConstant(EltIdx - 16, MVT::i8));
+      pshufbMask.push_back(DAG.getConstant(EltIdx, MVT::i8));
     }
     V2 = DAG.getNode(X86ISD::PSHUFB, dl, MVT::v16i8, V2,
                      DAG.getNode(ISD::BUILD_VECTOR, dl,