[Exegesis][RISCV] Remove the usage of RISCVSubtarget

Replace the usage of RISCVSubtarget in RISCVExegesisPreprocessingPass
with an equivalent. Because including RISCVSubtarget.h might slow down
the build speed by hindering the degree of parallelism for components
outside LLVMRISCVCodeGen.

Author: mshockwave

llvm/tools/llvm-exegesis/lib/RISCV/RISCVExegesisPreprocessing.cpp

@@ -12,9 +12,10 @@
 #include "RISCV.h"
 #include "RISCVExegesisPasses.h"
 #include "RISCVRegisterInfo.h"
-#include "RISCVSubtarget.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/TargetInstrInfo.h"
+#include "llvm/MC/MCContext.h"
 
 using namespace llvm;
 
@@ -63,10 +64,13 @@ static bool processAVLOperand(MachineInstr &MI, MachineRegisterInfo &MRI,
 
 bool RISCVExegesisPreprocessing::runOnMachineFunction(MachineFunction &MF) {
   MachineRegisterInfo &MRI = MF.getRegInfo();
-  const auto &STI = MF.getSubtarget<RISCVSubtarget>();
-  if (!STI.hasVInstructions())
+  // We could have use RISCVSubtarget::hasVInstructions here but including
+  // RISCVSubtarget.h would serialize the build of components outside
+  // LLVMRISCVCodeGen.
+  const MCSubtargetInfo &STI = *MF.getContext().getSubtargetInfo();
+  if (!STI.hasFeature(RISCV::FeatureStdExtZve32x))
     return false;
-  const TargetInstrInfo &TII = *STI.getInstrInfo();
+  const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
 
   LLVM_DEBUG(MF.print(dbgs() << "===Before RISCVExegesisPoreprocessing===\n");
              dbgs() << "\n");