[AArch64] Support reserving arbitrary general purpose registers

This is a follow up to D48580 and D48581 which allows reserving
arbitrary general purpose registers with the exception of registers
with special purpose (X8, X16-X18, X29, X30) and registers used by LLVM
(X0, X19). This change also generalizes some of the existing logic to
rely entirely on values generated from tablegen.

Differential Revision: https://reviews.llvm.org/D56305

llvm-svn: 353957