MIPS32: Fix string loads that use kDexCacheViaMethod.

This fixes some string-related crashes, in particular,
failures in test 563-checker-fakestring.

The bug was introduced in commit
aad75c6d.

Test: booted MIPS32R2 in QEMU
Test: test-art-target-run-test-optimizing (MIPS32R2) on CI20

Change-Id: Ie7d9a5b2c572b0933756f6e95c567968f811342c