[llvm] [AArch64][PAC] Expand blend(reg, imm) operation in aarch64-pauth pass (PR #74729)

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

This patch does not change [GlobalISel-specific implementation](https://github.com/llvm/llvm-project/blob/db3bc494875626c6b8e7392f08c631489b056702/llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp#L6763) in `AArch64InstructionSelector::selectIntrinsic`. Looks like custom code is only needed for `Intrinsic::ptrauth_blend` because of a bug in TableGen backend for GlobalISel. I expect #74492 to make that custom handling of `Intrinsic::ptrauth_blend` unneeded. (This patch itself is unaffected by the bug because `PAUTH_BLEND` instruction defines `GPR64:$disc` and the name `$disc` is not used by the pattern).

https://github.com/llvm/llvm-project/pull/74729