MIPS: Use sltiu instead of LoadConst32() + sltu

Bltu is sltu + bnez so we can use sltiu + bnez without
loading constant (if it is 16-bit constant).

Additionally, in VisitInvokeInterface() LoadConst32() is moved
to before Jalr(T9) so the load can be taken into the delay slot.

Test: ./testrunner.py --target --optimizing in QEMU

Change-Id: Ic19f251aeba015be38b7d3690e78b2fe59e7c5ae