[Hexagon] Remove unintended fallthrough from MC duplex code

[rnk]

Summary

I recently added these fall through annotations, but it's not clear if
the code is intended to fall through. In this patch, I changed them to
breaks, which seems more likely to be correct. If so, please approve
this patch and I will commit it. If not, I will reupload this patch to
remove the FIXME comments from the annotations.

[kparzysz]

Thanks!

llvm/lib/Target/Hexagon/MCTargetDesc/HexagonMCDuplexInfo.cpp

case HexagonII::HSIG_L1:
     switch (Gb) {
     default:
       break;
     case HexagonII::HSIG_L1:
       return 0;
     case HexagonII::HSIG_A:
       return 0x4;
     }
-    LLVM_FALLTHROUGH; // FIXME: Intentional?
+    break;
   case HexagonII::HSIG_L2:
     switch (Gb) {
     default:
       break;
     case HexagonII::HSIG_L1:
       return 0x1;
     case HexagonII::HSIG_L2:
       return 0x2;
     case HexagonII::HSIG_A:
       return 0x5;
     }
-    LLVM_FALLTHROUGH; // FIXME: Intentional?
+    break;
   case HexagonII::HSIG_S1:
     switch (Gb) {
     default:
       break;
     case HexagonII::HSIG_L1:
       return 0x8;
     case HexagonII::HSIG_L2:
       return 0x9;
     case HexagonII::HSIG_S1:
       return 0xA;
     case HexagonII::HSIG_A:
       return 0x6;
     }
-    LLVM_FALLTHROUGH; // FIXME: Intentional?
+    break;
   case HexagonII::HSIG_S2:
     switch (Gb) {
     default:
       break;
     case HexagonII::HSIG_L1:
       return 0xC;
     case HexagonII::HSIG_L2:
       return 0xD;
     case HexagonII::HSIG_S1:
       return 0xB;
     case HexagonII::HSIG_S2:
       return 0xE;
     case HexagonII::HSIG_A:
       return 0x7;
     }
-    LLVM_FALLTHROUGH; // FIXME: Intentional?
+    break;
   case HexagonII::HSIG_A:
     switch (Gb) {
     default:
       break;
     case HexagonII::HSIG_A:
       return 0x3;
     }
-    LLVM_FALLTHROUGH; // FIXME: Intentional?
+    break;
   case HexagonII::HSIG_Compound:
     switch (Gb) {
     case HexagonII::HSIG_Compound:
       return 0xFFFFFFFF;
     }
     break;
   }
   return 0xFFFFFFFF;