[AArch64]: Add support for parsing rN registers.

Summary:
Allow rN registers to be simply parsed as correspoing xN registers.
The "register ... asm("rN")" is an command to the
compiler's register allocator, not an operand to any individual assembly
instruction. GCC documents this syntax as "...the name of the register
that should be used."

This is needed to support the changes in Linux kernel (see
https://lkml.org/lkml/2018/3/1/268 )

Note: This will add support only for the limited use case of
register ... asm("rN"). Any other uses that make rN leak into assembly
are not supported.

Reviewers: kristof.beyls, rengolin, peter.smith, t.p.northover

Reviewed By: peter.smith

Subscribers: javed.absar, eraman, cfe-commits, srhines

Differential Revision: https://reviews.llvm.org/D44815

llvm-svn: 328829