The old implementation caused odd crashes on ARM, which were fixed by
changing a local variable type. The main suspect is the use of a static
local variable. This CL replaces the static local variable with
constexpr, which still ensures the compiler sees the expressions as
constants.

The CL also replaces Slice operations in the functions' inner loop with
iterator-style pointer operations, which can help the compiler generate
less code.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192832175