MIPS64: Ease test for using "Lui + Dinsu" to load 64-bit constants.

Test was unnecessarily flagging some negative "Low32Bits(value)"
values as requiring 3 instructions to load the 64-bit constant
"value" when those values really only require 2 instructions. This
prevents minimizing the corresponding instruction sequence created
by LoadConst64().

Test: mma test-art-host-gtest

Change-Id: Ib1490000ca15f5e0a23ad3e57d8ede90fd0d8f53