[WebAssembly] Add an LLVM_FALLTHROUGH to address a warning. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324777 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/WebAssembly/WebAssemblyFastISel.cpp

@@ -1165,6 +1165,7 @@ bool WebAssemblyFastISel::selectStore(const Instruction *I) {
   switch (getSimpleType(Store->getValueOperand()->getType())) {
   case MVT::i1:
     VTIsi1 = true;
+    LLVM_FALLTHROUGH;
   case MVT::i8:
     Opc = WebAssembly::STORE8_I32;
     break;