Fix clang -Wimplicit-fallthrough warnings across llvm, NFC

This patch should not introduce any behavior changes. It consists of
mostly one of two changes:
1. Replacing fall through comments with the LLVM_FALLTHROUGH macro
2. Inserting 'break' before falling through into a case block consisting
   of only 'break'.

We were already using this warning with GCC, but its warning behaves
slightly differently. In this patch, the following differences are
relevant:
1. GCC recognizes comments that say "fall through" as annotations, clang
   doesn't
2. GCC doesn't warn on "case N: foo(); default: break;", clang does
3. GCC doesn't warn when the case contains a switch, but falls through
   the outer case.

I will enable the warning separately in a follow-up patch so that it can
be cleanly reverted if necessary.

Reviewers: alexfh, rsmith, lattner, rtrieu, EricWF, bollu

Differential Revision: https://reviews.llvm.org/D53950

llvm-svn: 345882
diff --git a/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp b/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
index 478a473..7769a35 100644
--- a/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
+++ b/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
@@ -236,6 +236,7 @@
       // MI will become a KILL, don't considers it in scheduling
       return AluDiscarded;
     }
+    break;
   default:
     break;
   }