[llvm] [AArch64][PAC] Implement code generation for @llvm.ptrauth.auth (PR #72502)

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

A few notes:
* DummyLoad checker was fixed to use the register passed in argument of `checkAuthenticatedRegister` instead of LR, the operands of BRK instruction are updated to closely follow the numbers already used in @ahmedbougacha's branch
* this patch has similar purpose to https://reviews.llvm.org/D132386, but pseudo instructions are generated by TableGen-erated code plus custom inserter and expanded before AsmPrinter
* for uniformity, this patch computes a blend with one of the arguments being zero differently from the non-blended case:
  - `blend(value, 0)` clears top 16 bits of `value` (as it is done for any other integer modifier)
  - `blend(NULL, n)` shifts `n` left by 48 bits (as it is done for any other pointer)

https://github.com/llvm/llvm-project/pull/72502