Try to split constant XORs into two shiftimms on ARM.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26448 3ad0048d-3df7-0310-abae-a5850022a9f2
Reformat and comment is_thumb32_imm

I've just regrouped the conditions a bit for easier understanding. No
functional changes.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26449 3ad0048d-3df7-0310-abae-a5850022a9f2