Pull in r300404 from upstream llvm trunk (by me):

Pull in r300404 from upstream llvm trunk (by me):

  Use correct registers for "A" inline asm constraint

  Summary:
  In PR32594, inline assembly using the 'A' constraint on x86_64 causes
  llvm to crash with a "Cannot select" stack trace.  This is because
  `X86TargetLowering::getRegForInlineAsmConstraint` hardcodes that 'A'
  means the EAX and EDX registers.

  However, on x86_64 it means the RAX and RDX registers, and on 16-bit
  x86 (ia16?) it means the old AX and DX registers.

  Add new register classes in `X86RegisterInfo.td` to support these
  cases, and amend the logic in `getRegForInlineAsmConstraint` to cope
  with different subtargets.  Also add a test case, derived from
  PR32594.

  Reviewers: craig.topper, qcolombet, RKSimon, ab

  Reviewed By: ab

  Subscribers: ab, emaste, royger, llvm-commits

  Differential Revision: https://reviews.llvm.org/D31902

This should fix crashes when using the 'A' constraint on amd64, for
example as it is being used in Xen.

Reported by:	royger
MFC after:	3 days