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=

Review URL: https://codereview.chromium.org/14257006
Patch from Dusan Milosavljevic <Dusan.Milosavljevic@rt-rk.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14426 ce2b1a6d-e550-0410-aec6-3dcde31c8c00