[AArch64] Fix fastcc -tailcallopt epilog code generation.

Summary:
Fix a bug in epilog generation where the incoming stack arguments were
not being popped for fastcc functions when -tailcallopt was passed.

Reviewers: t.p.northover, mcrosier, jmolloy, rengolin

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D16894

llvm-svn: 261650