[ARM GlobalISel] Support G_CONSTANT for Thumb2

All we have to do is mark it as legal.

This allows us to select a lot of new patterns handled by TableGen. This
patch adds tests for them and splits up the existing test file for
binary operators into 2 files, one for arithmetic ops and one for
logical ones.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349610 91177308-0d34-0410-b5e6-96231b3b80d8