[AArch64] Implement add/adds/sub/subs/cmp/cmn with negative immediate aliases

[AArch64] Implement add/adds/sub/subs/cmp/cmn with negative immediate aliases

This patch teaches the AsmParser to accept add/adds/sub/subs/cmp/cmn
with a negative immediate operand and convert them as shown:

  add  Rd, Rn, -imm -> sub  Rd, Rn, imm
  sub  Rd, Rn, -imm -> add  Rd, Rn, imm
  adds Rd, Rn, -imm -> subs Rd, Rn, imm
  subs Rd, Rn, -imm -> adds Rd, Rn, imm
  cmp  Rn, -imm     -> cmn  Rn, imm
  cmn  Rn, -imm     -> cmp  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 (gas). They are documented in the "ARMv8 Instruction Set
Overview", in the "Arithmetic (immediate)" section. This makes llvm-mc
a programmer-friendly assembler !

This also fixes PR20978: "Assembly handling of adding negative numbers
not as smart as gas".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241166 91177308-0d34-0410-b5e6-96231b3b80d8