The issue with slow compile-time was caused by an assert in
AArch64RegisterInfo.cpp. The assert invokes 'checkAllSuperRegsMarked'
after adding all the reserved registers. This call gets very expensive
after adding the _HI registers due to the way the function searches
in the 'Exception' list, which is expected to be a small list but isn't
(the patch added 190 _HI regs).
It was possible to rewrite the code in such a way that the _HI registers
are marked as reserved after the check. This makes the problem go away
entirely and restores compile-time to what it was before (tested for
`check-runtimes`, which previously showed a ~5x slowdown).
This reverts commits:
1434d2ab215e3ea9c5f34689d056edd3d4423a78
2704647fb7986673b89cef1def729e3b022e2607
318c69de52