[RegisterScavenging] Add asserts for better errors

[sebastian-ne]

Summary

These cases were failing before, but with cryptic asserts.
Add asserts in the RegScavenger that fail earlier with better
messages. NFC

[arsenm]

llvm/lib/CodeGen/RegisterScavenging.cpp

430

What if you did intend to scavenge at the beginning? Also seems weird to assert here when there's a break on begin above

[sebastian-ne]

llvm/lib/CodeGen/RegisterScavenging.cpp

430

I tried to scavenge a register with scavengeRegisterBackwards while the scavenger was in forward mode.
The break above gets only hit when FoundTo == true, so when I == To. If we never find To while iterating backwards from From (because To is somewhere down in the basic block), we will never break and it will crash when trying to access an invalid I (the error message is something like Assertion failed: (!NodePtr->isKnownSentinel())).

llvm/lib/CodeGen/RegisterScavenging.cpp

findSurvivorBackwards(const MachineRegisterInfo &MRI,
   MCPhysReg Survivor = 0;
   MachineBasicBlock::iterator Pos;
   MachineBasicBlock &MBB = *From->getParent();
   unsigned InstrLimit = 25;
   unsigned InstrCountDown = InstrLimit;
   const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
   LiveRegUnits Used(TRI);
 
+  assert(From->getParent() == To->getParent() &&
+         "Target instruction is in other than current basic block, use "
+         "enterBasicBlockEnd first");
+
   for (MachineBasicBlock::iterator I = From;; --I) {
     const MachineInstr &MI = *I;
 
     Used.accumulate(MI);
 
     if (I == To) {
       // See if one of the registers in RC wasn't used so far.
       for (MCPhysReg Reg : AllocationOrder) {
if (FoundTo) {
       }
       if (FoundVReg) {
         InstrCountDown = InstrLimit;
         Pos = I;
       }
       if (I == MBB.begin())
         break;
     }
+    assert(I != MBB.begin() && "Did not find target instruction while "
+                               "iterating backwards");
   }
 
   return std::make_pair(Survivor, Pos);
 }
 
 static unsigned getFrameIndexOperandNum(MachineInstr &MI) {
   unsigned i = 0;
   while (!MI.getOperand(i).isFI()) {