[AArch64] Add logical alias instructions to MC AsmParser

This patch teaches the AsmParser to accept some logical+immediate
instructions and convert them as shown:

  bic  Rd, Rn, #imm  ->  and Rd, Rn, #~imm
  bics Rd, Rn, #imm  ->  ands Rd, Rn, #~imm
  orn  Rd, Rn, #imm  ->  orr Rd, Rn, #~imm
  eon  Rd, Rn, #imm  ->  eor Rd, Rn, #~imm

Those instructions are an alternate syntax available to assembly coders,
and are needed in order to support code already compiling with some other
assemblers. For example, the bic construct is used by the linux kernel.