[llvm] [AArch64] Resolve FIXME: Use scavengeRegisterBackwards to find the best unused register (PR #78910)

[via llvm-commits]

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@@ -11,34 +12,148 @@
 # - other direct branches don't seem to be generated by the AArch64 codegen
 --- |
   define void @nobranch_fallthrough(i32 %a, i32 %b) speculative_load_hardening {
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qcolombet wrote:

Could you add a comment of what needs to happen or must not happen in the comment?

The manual checks were conveying this in a more direct way (but broken in its own way :P).

https://github.com/llvm/llvm-project/pull/78910