[ARM] Mark labels in skipAlignedDPRCS2Spills as fallthrough (NFC).

The comment at the top of the switch statement indicates that the
fall-through behavior is intentional. By using LLVM_FALLTHROUGH,
-Wimplicit-fallthrough are silenced, which is enabled by default in GCC
7.

llvm-svn: 309272
diff --git a/llvm/lib/Target/ARM/ARMFrameLowering.cpp b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
index 16b54e8..b204faa 100644
--- a/llvm/lib/Target/ARM/ARMFrameLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
@@ -1283,9 +1283,11 @@
   case 7:
     ++MI;
     assert(MI->mayStore() && "Expecting spill instruction");
+    LLVM_FALLTHROUGH;
   default:
     ++MI;
     assert(MI->mayStore() && "Expecting spill instruction");
+    LLVM_FALLTHROUGH;
   case 1:
   case 2:
   case 4: