[AArch64] Fix scavenged spill slot base when stack realignment required.

Summary:
Use the FP for scavenged spill slot accesses to prevent corruption of
the callee-save region when the SP is re-aligned.

Based on problem and patch reported by @paulwalker-arm

This is an alternative to solution proposed in D45770

Reviewers: t.p.northover, paulwalker-arm, thegameg, javed.absar

Subscribers: qcolombet, mcrosier, paulwalker-arm, kristof.beyls, rengolin, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D46063

llvm-svn: 330976