[ARM] Clean up choice of narrow instructions in ARMAsmParser, NFC

This patch makes a couple of changes to how we decide whether to use the narrow
or wide encoding of thumb2 instructions:
 * Common out the detection of the .w qualifier
 * Check for the CPSR operand in a consistent way

Differential Revision: https://reviews.llvm.org/D34460

llvm-svn: 305992