A64: Remove two TODOs that will not be done.

A64: Remove two TODOs that will not be done.

- For the LDP/STP TODO: The arguments need dereferencing, so using LDP/STP would
  require some additional checks.
- For the constant pools TODO: Unless we need to, there is no benefit from
  trying to generate constant pools more often, as it would likely only pollute
  the I-cache.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/151363004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19316 ce2b1a6d-e550-0410-aec6-3dcde31c8c00