[RISCV] Fix std::advance slowness

[apazos]

Summary

It seems std::advance template is treating "-MFI.getCalleeSavedInfo().size()" as a large unsigned value", causing slowness.

Thanks to Henrik Gustafsson for reporting the issue.

[asb]

Thanks, looks good to me.

[asb]

Actually, how about replacing those two lines with auto LastFrameDestroy = std::prev(MBBI, MFI.getCalleeSavedInfo().size())?

llvm/trunk/lib/Target/RISCV/RISCVFrameLowering.cpp

void RISCVFrameLowering::emitEpilogue(MachineFunction &MF,
   auto *RVFI = MF.getInfo<RISCVMachineFunctionInfo>();
   DebugLoc DL = MBBI->getDebugLoc();
   unsigned FPReg = getFPReg(STI);
   unsigned SPReg = getSPReg(STI);
 
   // Skip to before the restores of callee-saved registers
   // FIXME: assumes exactly one instruction is used to restore each
   // callee-saved register.
-  MachineBasicBlock::iterator LastFrameDestroy = MBBI;
-  std::advance(LastFrameDestroy, -MFI.getCalleeSavedInfo().size());
+  auto LastFrameDestroy = std::prev(MBBI, MFI.getCalleeSavedInfo().size());
 
   uint64_t StackSize = MFI.getStackSize();
 
   // Restore the stack pointer using the value of the frame pointer. Only
   // necessary if the stack pointer was modified, meaning the stack size is
   // unknown.
   if (RI->needsStackRealignment(MF) || MFI.hasVarSizedObjects()) {
     assert(hasFP(MF) && "frame pointer should not have been eliminated");