[llvm] [RISCV][NewPM] Port RISCVCodeGenPrepare to the new pass manager (PR #168381)

[Alex Bradbury via llvm-commits]

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@@ -26,8 +26,16 @@ class RISCVRegisterBankInfo;
 class RISCVSubtarget;
 class RISCVTargetMachine;
 
-FunctionPass *createRISCVCodeGenPreparePass();
-void initializeRISCVCodeGenPreparePass(PassRegistry &);
+class RISCVCodeGenPreparePass : public PassInfoMixin<RISCVCodeGenPreparePass> {
+private:
+  const RISCVTargetMachine &TM;
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asb wrote:

I've made that change. FWIW this is where I've followed in the footsteps of the AMDGPU passes.

CC @arsenm 

https://github.com/llvm/llvm-project/pull/168381