[GlobalISel] Fix a warning

This patch fixes:

  llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h:1097:7:
  error: unannotated fall-through between switch labels
  [-Werror,-Wimplicit-fallthrough]

Author: kazutakahirata

llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h

@@ -1094,6 +1094,7 @@ class LegalizationArtifactCombiner {
         if (!OpTy.isVector() || !LI.isLegal({TargetOpcode::G_UNMERGE_VALUES,
                                              {DestTy, SrcUnmergeSrcTy}}))
           return false;
+        break;
       case LegalizeActions::Lower:
       case LegalizeActions::Unsupported:
         break;