[ARM] Extract some code from ARMConstantIslandPass

Create the ARMBasicBlockUtils class for tracking and querying basic
blocks sizes so we can use them when generating low-overhead loops.

Differential Revision: https://reviews.llvm.org/D63265

llvm-svn: 363530
GitOrigin-RevId: 966f4e874e069b64868e3cb818e926ac8c8f3107