Replace "fallthrough" comments with LLVM_FALLTHROUGH

This is a mechanical change of comments in switches like fallthrough,
fall-through, or fall-thru to use the LLVM_FALLTHROUGH macro instead.

llvm-svn: 278902
diff --git a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
index 657a045..76aa7bf 100644
--- a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
+++ b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
@@ -1413,7 +1413,7 @@
     // immediates here as the normal path expects bit 31 to be sign extended.
     if (Idx == 1 && Imm.getBitWidth() == 64 && isUInt<32>(Imm.getZExtValue()))
       return TTI::TCC_Free;
-    // Fallthrough
+    LLVM_FALLTHROUGH;
   case Instruction::Add:
   case Instruction::Sub:
   case Instruction::Mul: