[mips][fastisel] Correct argument lowering for (f64, f64, i32) and similar.

Summary:
Allocating an AFGR64 shadows two GPR32's instead of just one.

This fixes an LNT regression detected by our internal buildbots.

Reviewers: sdardis

Subscribers: dsanders, sdardis, llvm-commits

Differential Revision: https://reviews.llvm.org/D23012

llvm-svn: 277348