X86: Allow conditional tail calls in Win64 "leaf" functions (PR26302)

We can't use Jcc to leave a Win64 function in general, because that
confuses the unwinder. However, for "leaf" functions, that is, functions
where the return address is always on top of the stack and which don't
have unwind info, it's OK.

Differential Revision: https://reviews.llvm.org/D24836

llvm-svn: 282920