[clang] [llvm] [X86] support reserve r8~r15 on X86_64 (PR #180242)

[Phoebe Wang via llvm-commits]

================
@@ -0,0 +1,93 @@
+;; Check if manually reserved registers are always excluded from being saved by
+;; the function prolog/epilog, even for callee-saved ones, as per GCC behavior.
----------------
phoebewang wrote:

We only have a few callee-saved registers what's the expected behavior for caller-save ones?

https://github.com/llvm/llvm-project/pull/180242