Mark intentional fall-through with LLVM_FALLTHROUGH.

... to silence gcc 7's default -Wimplicit-fallthrough.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317573 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/GlobalISel/LegalizerInfo.cpp

@@ -279,6 +279,7 @@ LegalizerInfo::findAction(const SizeAndActionsVec &Vec, const uint32_t Size) {
     // Special case for scalarization:
     if (Vec == SizeAndActionsVec({{1, FewerElements}}))
       return {1, FewerElements};
+    LLVM_FALLTHROUGH;
   case NarrowScalar: {
     // The following needs to be a loop, as for now, we do allow needing to
     // go over "Unsupported" bit sizes before finding a legalizable bit size.