[llvm] [llvm][AArch64] Preserve regmask when expanding the BLR_BTI pseudo instruction (PR #73927)

[David Spickett via llvm-commits]

DavidSpickett wrote:

With this fix the assembly is:
```
        mov     w19, w0
        bl      _setjmp
        hint    #36
        cbz     w19, .LBB0_2
```
Because we now keep `csr_aarch64_aapcs` around (w19 is callee saved).

https://github.com/llvm/llvm-project/pull/73927