[Arm] Various cleanups to the Arm assembler backend.

[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

Committed: https://code.google.com/p/v8/source/detail?r=21932

[rmcilroy, 2014-06-04 12:33:24]

PTAL, thanks.

[ulan, 2014-06-05 07:08:57]

lgtm

[rmcilroy, 2014-06-23 11:27:37]

Committed patchset #1 manually as r21932 (presubmit successful).