ARM IAS: account for predicated pre-UAL mnemonics

Checking the trailing letter of the mnemonic is insufficient.  Be more thorough
in the scanning of the instruction to ensure that we correctly work with the
predicated mnemonics.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198235 91177308-0d34-0410-b5e6-96231b3b80d8