MIPS: Fix polymorphic invokes.

This fixes MIPS-specific optimizing compiler bugs introduced in
commit ac141397.

The below tests still fail in interpreter mode and this will be
addressed separately.

Test: booted MIPS32R2 in QEMU
Test: ./run-test --optimizing 953-invoke-polymorphic-compiler
Test: ./run-test --optimizing 955-methodhandles-smali
Test: ./run-test --optimizing 956-methodhandles
Test: ./run-test --optimizing 957-methodhandle-transforms
Test: ./run-test --optimizing 958-methodhandle-emulated-stackframe
Test: ./run-test --optimizing 959-invoke-polymorphic-accessors
Test: booted MIPS64 (with 2nd arch MIPS32R6) in QEMU
Test: same tests as above and
Test: ./run-test --optimizing --64 953-invoke-polymorphic-compiler
Test: ./run-test --optimizing --64 955-methodhandles-smali
Test: ./run-test --optimizing --64 956-methodhandles
Test: ./run-test --optimizing --64 957-methodhandle-transforms
Test: ./run-test --optimizing --64 958-methodhandle-emulated-stackframe
Test: ./run-test --optimizing --64 959-invoke-polymorphic-accessors

Change-Id: I6119e8b86448e022f68b10cd3a8912a2bd1976c5