MIPS64: Fix incorrect register in EmitIsStringWrapperSafeForDefaultValue().

TEST: mjsuint/regress/regress-760-1
MIPS64: Fix Smi handle in GenerateNegativeLookup().

TEST: mjsunit/regress/regress-2980
MIPS64: Fix store -Infinity to HeapNumber.

TEST: msjunit/regress/regress-2056
MIPS64: Wrong load number of output registers.

TEST: mjsunit/regress/regress-1748