ARMAsmParser: clean up of isImmediate functions

- we are now using immediate AsmOperands so that the range check functions are
  tablegen'ed.
- Big bonus is that error messages become much more accurate, i.e. instead of a
  useless "invalid operand" error message it will not say that the immediate
  operand must in range [x,y], which is why regression tests needed updating.

More tablegen operand descriptions could probably benefit from using
immediateAsmOperand, but this is a first good step to get rid of most of the
nearly identical range check functions. I will address the remaining immediate
operands in next clean ups.

Differential Revision: https://reviews.llvm.org/D31333

llvm-svn: 299358