[PATCH] [mips] Reserve $at and use it for frame pointer elimination.

[Vladimir Stefanovic]

Hi dsanders, petarj,

When spilling a register, RegScavenger calls MipsSERegisterInfo::eliminateFI(),
which needs a free register to handle offsets over 16 bits, resulting in
"Cannot scavenge register without an emergency spill slot!" assert.
This patch puts $at on the reserved registers list, and uses it in eliminateFI()
instead of requesting a register from RegScavenger.

http://reviews.llvm.org/D6509

Files:
  lib/Target/Mips/MipsRegisterInfo.cpp
  lib/Target/Mips/MipsSERegisterInfo.cpp
  test/CodeGen/Mips/vector-multiply.ll
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