Move PC-relative addressing bases to a better position.

Move the platform-specific HX86ComputeBaseMethodAddress and
HArmDexCacheArraysBase to the latest dominator of their uses
outside any loop. This brings the base closer to the first
use (previously, it was in the entry block) and relieves
some pressure on the register allocator while avoiding
recalculation of the base in a loop.

Change-Id: I231aa81eb5b4de9af2d0167054d06b65eb18a636