[RISCV] Split the pseudo instruction splitting pass

Extracts the atomic pseudo-instructions' splitting from `riscv-expand-pseudo`
/ `RISCVExpandPseudo` into its own pass, `riscv-expand-atomic-pseudo` /
`RISCVExpandAtomicPseudo`. This allows for the expansion of atomic operations
to continue to happen late (the new pass is added in `addPreEmitPass2`, so
those expansions continue to happen in the same place), while the remaining
pseudo-instructions can now be expanded earlier and benefit from more
optimization passes. The nonatomics pass is now added in `addPreSched2`.

Differential Revision: https://reviews.llvm.org/D79635

Author: luismarques

llvm/lib/Target/RISCV/CMakeLists.txt

@@ -19,6 +19,7 @@ add_public_tablegen_target(RISCVCommonTableGen)
 add_llvm_target(RISCVCodeGen
   RISCVAsmPrinter.cpp
   RISCVCallLowering.cpp
+  RISCVExpandAtomicPseudoInsts.cpp
   RISCVExpandPseudoInsts.cpp
   RISCVFrameLowering.cpp
   RISCVInstrInfo.cpp

llvm/lib/Target/RISCV/RISCV.h

@@ -43,6 +43,9 @@ void initializeRISCVMergeBaseOffsetOptPass(PassRegistry &);
 FunctionPass *createRISCVExpandPseudoPass();
 void initializeRISCVExpandPseudoPass(PassRegistry &);
 
+FunctionPass *createRISCVExpandAtomicPseudoPass();
+void initializeRISCVExpandAtomicPseudoPass(PassRegistry &);
+
 InstructionSelector *createRISCVInstructionSelector(const RISCVTargetMachine &,
                                                     RISCVSubtarget &,
                                                     RISCVRegisterBankInfo &);