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

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

Ping.

Notes:
* this patch is arranged so that #72502 can reuse `emitBlend` method while `expandPAuthBlend` merely translates `PAUTH_BLEND` pseudo instruction (compared to #72502, added a dedicated method for uniformity)
* on enabling PAUTH_BLEND on cores without FEAT_PAUTH: in the [discussion of the original patch](https://reviews.llvm.org/D132384#3879529) on Phabricator, there were no strict opinions for either enabling or disabling it. Wouldn't enabling blend operation (and, afterwards, sign and auth using I-keys) useful for something like runtime libraries?

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