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

llvm-svn: 300404