[mips] Fix spill slot for mips3, n64 abi

When program is compiled for mips3 with n64 abi, wrong register class
is used for creating an emergency spill slot. This patch fixes the
correct register class to be chosen.

This patch resolves PR35859.

Thanks to John Baldwin for reporting the issue!

Differential Revision: https://reviews.llvm.org/D47938

llvm-svn: 334419