[ConstantFolding][X86] Replace an LLVM_FALLTHROUGH with a break because it...

[ConstantFolding][X86] Replace an LLVM_FALLTHROUGH with a break because it really shouldn't fallthrough.

This is actually NFC because the next case starts with the same if statement as this case did. So the result will be the same and it will fallthrough to the end of the switch. But there's no reason to rely on that so we should just break.

llvm-svn: 304680