[PATCH] [AArch64] Allow negative immediates for ADD/SUB in asm

[Renato Golin]

Hi t.p.northover, jmolloy, john.brawn,

Allow assembly code (either in an asm file or inline asm) to have negative
offsets for ADD/SUB, and convert them to their inverse instructions as per
the ARMv8 Instruction Set Overview, section 5.4.1.

I'm not really sure of a few things:
1. If I accept negative numbers in isAddSubImm(), will this break other stuff?
2. The manual doesn't say specifically, but I'm assuming we can't swap ADDS/SUBS.
3. The error message may not be in the best of places, nor the best of texts. I'm open to suggestions.
4. Is this really the best way to modify the MCInst?
5. Adding support for #-4096 and higher powers of two may be trivial. I'll check while this review goes on.
6. Changing the parser like this seems a lot better than both printers (as I had in bugzilla).

Fixes PR20978.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D10810

Files:
  lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
  test/MC/AArch64/addsub-inline-asm.ll
  test/MC/AArch64/addsub.s
  test/MC/AArch64/basic-a64-diagnostics.s

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