[llvm] [AMDGPU] Support preloading hidden kernel arguments (PR #98861)

[Matt Arsenault via llvm-commits]

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@@ -5856,6 +5860,11 @@ may insert a trap instruction at the start of the kernel prologue to manage
 situations where kernarg preloading is attempted on hardware with incompatible
 firmware.
 
+With code object V5 and later, hidden kernel arguments that are normally accessed
+through the Implicit Argument Ptr, may be preloaded into User SGPRs. These
+arguments are added to the kernel function signature and are marked with the
+attribute "amdgpu-hidden-argument". (See :ref:`amdgpu-llvm-ir-attributes-table`).
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arsenm wrote:

Also inreg 

https://github.com/llvm/llvm-project/pull/98861