MIPS32: Move code from RA to T9 in longjump

Do not use the return address. It should not be clobbered as we
may need it, e.g., for single-frame deopt.

This fixes following tests:
* 449-checker-bce
* 534-checker-bce-deoptimization
* 535-deopt-and-inlining

Change-Id: Ic94a2897c7336da094a8937ab1bc4a02a680de53
diff --git a/runtime/arch/mips/context_mips.cc b/runtime/arch/mips/context_mips.cc
index 4dedb33..375a03a 100644
--- a/runtime/arch/mips/context_mips.cc
+++ b/runtime/arch/mips/context_mips.cc
@@ -28,11 +28,11 @@
   std::fill_n(gprs_, arraysize(gprs_), nullptr);
   std::fill_n(fprs_, arraysize(fprs_), nullptr);
   gprs_[SP] = &sp_;
-  gprs_[RA] = &ra_;
+  gprs_[T9] = &t9_;
   gprs_[A0] = &arg0_;
   // Initialize registers with easy to spot debug values.
   sp_ = MipsContext::kBadGprBase + SP;
-  ra_ = MipsContext::kBadGprBase + RA;
+  t9_ = MipsContext::kBadGprBase + T9;
   arg0_ = 0;
 }