[AArch64] Do not allocate unnecessary emergency slot.

Fix an issue where the compiler still allocates an emergency spill slot even
though it already decided to spill an extra callee-save register to use
as a scratch register.

Reviewers: gberry, thegameg, mstorsjo, t.p.northover

Reviewed By: thegameg

Differential Revision: https://reviews.llvm.org/D65504

llvm-svn: 367540