[AArch64][PAC] Introduce AArch64::PAC pseudo instruction (#146488) Introduce a pseudo instruction carrying address and immediate modifiers as separate operands to be selected instead of a pair of `MOVKXi` and `PAC[ID][AB]` . The new pseudo instruction is expanded in AsmPrinter, so that `MOVKXi` is emitted immediately before `PAC[ID][AB]`. This way, an attacker cannot control the immediate modifier used to sign the value, even if address modifier can be substituted. To simplify the instruction selection, select `AArch64::PAC` pseudo using TableGen pattern and post-process its `$AddrDisc` operand by custom inserter hook - this eliminates duplication of the logic for DAGISel and GlobalISel. Furthermore, this improves cross-BB analysis in case of DAGISel.
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