ARM clean up the imm_sr operand class representation.

ARM clean up the imm_sr operand class representation.

Represent the operand value as it will be encoded in the instruction. This
allows removing the specialized encoder and decoder methods entirely. Add
an assembler match class while we're at it to lay groundwork for parsing the
thumb shift instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137879 91177308-0d34-0410-b5e6-96231b3b80d8