MIPS: Fix NaN handling for start index in ArraySplice.

Casting NaN to int is unpredictable, on different architectures it produces different int value.

TEST=test262/S15.4.4.10_A2.1_T2, S15.4.4.10_A2.2_T2, S15.4.4.12_A2.1_T2

BUG=