MIPS64: Comment SharedFunctionInfo::kFormalParameterCountOffset usage.

The FormalParameterCount (arg count) is a Smi on 32-bits, and is a
int32_t on 64-bit platforms. This will require special handling
if/when we do a unified port. Added a TODO to all usages.

No code change.
MIPS64 backport: Fix dictionary element load to pass correct elements kind.

Backport r19387, and adapt for mips64.

Using FAST_SMI_ELEMENTS triggers optimization on 64-bit architectures that load
only the higher 32 bits of the element. If the element is a pointer to undefined
that has 0 in the higher half than it is erroneously treated as SMI 0.

BUG=v8:3158
LOG=N
TEST=mjsunit/sparse-array-reverse,mjsunit/regress/regress-3158.js
R=danno@chromium.org, ishell@chromium.org

Review URL: https://codereview.chromium.org/166653005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19387 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
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