MIPS64: Fix wrong use ld, should be lw.

TEST: mjsunit/call-non-function
MIPS64: Change lw() to lwu() for unsigned operands.
MIPS64: Fix GetRelocateValue with new li 64-bit value.
MIPS64: Representation fix for DoLoadKeyedFixedArray()

When the representation is Integer32, we need to load this Smi
as an integer, effectively untagging it during the load.

TEST=mjsunit/keyed-ic, mjsunit/keyed-call-ic, mjsunit/arguments
(and many others)