[Arm] Various cleanups to the Arm assembler backend.

A couple of cleanups to the Arm backend to enable support of extended
OOL constant pools in a following CL.

 - Remove instruction pattern extern const's and replace their use with IsXXX()
   functions.
 - Do calculation of the target address of a load from constant pool in one
   place.
 - A couple of other small cleanups.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/317653003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21932 ce2b1a6d-e550-0410-aec6-3dcde31c8c00