MIPS64: Port `Reland [arm64] Improve some new builtins.`

Port a1103a11

Original commit message:
 - Simplify the variable-length pop sequence on entry. (It now uses
   smaller code with no branches.)
 - Use conditional compare to merge branches where appropriate.
 - Make use of Ldrsw + UntagSmiFieldMemOperand to load smis more
   efficiently.
 - Only load 'undefined' and 'null' once per builtin.
 - A few other small improvements.

Review-Url: https://codereview.chromium.org/2507683003
Cr-Commit-Position: refs/heads/master@{#41113}