[llvm] [AArch64] Fix failure with inline asm and svcount (PR #112537)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

The real problem here is that we shouldn't have a register class that contains mutually exclusive registers.  That said, the PR itself looks sounds and fixes a bug so I'm happy to accept.

I'll investigate the register class side of things separately and revisit this code to see if my assertion is correct.

https://github.com/llvm/llvm-project/pull/112537