MIPS64: Fix LCodeGen::DoFlooringDivByPowerOf2I() and MacroAssembler::TruncatingDiv().

TEST: mjsunit/compiler/division-by-constant
MIPS64: Fix DoLoadKeyedFixedArray() and DoStoreKeyedFixedArray().

TEST: mjsunit/array-iteration mjsunit/array-indexing etc.. (debug mode).
MIPS64: Fix FillRegisters() in SimulatorHelper.

This was correctly implmented for the hardware sampler using
V8_HOST_ARCH_MIPS64, but had not been fixed for simulator helper
running as V8_TARGET_ARCH_MIPS64.