MIPS64: Refactor implicit null checks in array/field get/set

Rationale: on MIPS64 64-bit loads and stores may be performed
as pairs of 32-bit loads/stores. Implicit null checks must be
associated with the first 32-bit load/store in a pair and not
the last. This change ensures proper association of said checks
(a few were done after the last 32-bit load/store in a pair)
and lays ground for further improvements in array/field get/set.

Additionally ported to MIPS32.

Test: mma test-art-target-run-test in QEMU
Test: mma test-art-host-gtest

Change-Id: If2612df62c21522959e69c637a36cc4ea962a32e