[WebAssembly] Update calls to computeKnownBits after the changes from r301620.

I didn't realize WebAssembly wasn't a default build target so I missed that changes were needed.

llvm-svn: 301629
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
index a67137f..257f1d1 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp
@@ -18,6 +18,7 @@
 #include "llvm/CodeGen/SelectionDAGISel.h"
 #include "llvm/IR/Function.h" // To access function attributes.
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;