[llvm] [AMDGPU] Initialize FrameOffsetReg for amdgpu_cs_chain functions (PR #165518)

[Matt Arsenault via llvm-commits]

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@@ -98,6 +98,7 @@ SIMachineFunctionInfo::SIMachineFunctionInfo(const Function &F,
     // set one up. For now, we can use s32 to match what amdgpu_gfx functions
     // would use if called, but this can be revisited.
     // FIXME: Only reserve this if we actually need it.
+    FrameOffsetReg = AMDGPU::SGPR33;
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arsenm wrote:

It still needs to compile without verifier error if it's UB. You don't have the option of rejecting a frame pointer, you can explicitly request it (and things like dynamic alloca force it too)

https://github.com/llvm/llvm-project/pull/165518