Reland "[WebAssembly] Expand setcc of v2i64"

This reverts commit e5cae5692b5899631b5bfe5c23234deb5efda10c, which
reverted 11850a6305c5778b180243eb06aefe86762dd4ce. The original revert
was done because of breakage that was actually in a separate commit,
2ab1b8c1ec452fb743f6cc5051e75a01039cabfe, which was also reverted and
has since been fixed and relanded.
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.h b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.h
index 4e0f7cf..58e088a 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.h
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.h
@@ -116,6 +116,7 @@
   SDValue LowerSIGN_EXTEND_INREG(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG) const;
+  SDValue LowerSETCC(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerAccessVectorElement(SDValue Op, SelectionDAG &DAG) const;
   SDValue LowerShift(SDValue Op, SelectionDAG &DAG) const;
 };