X86: Emit an ABI compliant prologue and epilogue for Win64

Win64 has specific contraints on what valid prologues and epilogues look
like.  This constraint is born from the flexibility and descriptiveness
of Win64's unwind opcodes.

Prologues previously emitted by LLVM could not be represented by the
unwind opcodes, preventing operations powered by stack unwinding to
successfully work.

Differential Revision: http://reviews.llvm.org/D7520

llvm-svn: 228641