[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.

0 files changed, 0 insertions, 0 deletions