[arm64] Improve some new builtins.

- 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.

BUG=

Review URL: https://codereview.chromium.org/1537903004

Cr-Commit-Position: refs/heads/master@{#32975}