[ARM GlobalISel] Minor refactoring. NFCI

Refactor the ARMInstructionSelector to cache some opcodes in the
constructor instead of checking all the time if we're in ARM or Thumb
mode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349143 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed