X86: Stop dereferencing end() in X86FrameLowering::emitEpilogue

On a Windows build of Chromium, r278532 (up to r278539)
X86FrameLowering::emitEpilogue because it wasn't wary enough of the
return of MachineBasicBlock::getFirstTerminator.  Guard all the uses
here.

Note that r278532 *looks* like an NFC commit (just an API change), but
it removes a couple of layers of abstraction and is probably causing
optimization differences in MSVC.

llvm-svn: 278572