MIPS64: Add debug check that optimized Smi store is done to entry containing Smi.

Modeled largely after r19389/5013ef84 from ishell@chromium.org.
MIPS64: Prevent 32-bit Smi load/store optimization on 32-bit platforms.

This should prevent the mips64 Smi ops that load/store from upper
half of Smi from being used on 32-bit port later.

Taken from r20634 / c87cac39 by haitao.feng@intel.com.
MIPS64: Fix QuietSignalingNaNs for MIPS64.

TEST: cctest/test-api/QuietSignalingNaNs
      cctest/test-api/Thread1