[RegisterScavenging] Add asserts for better errors

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

Differential Revision: https://reviews.llvm.org/D100109

llvm/lib/CodeGen/RegisterScavenging.cpp

@@ -369,6 +369,10 @@ findSurvivorBackwards(const MachineRegisterInfo &MRI,
   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;
 
@@ -423,6 +427,8 @@ findSurvivorBackwards(const MachineRegisterInfo &MRI,
       if (I == MBB.begin())
         break;
     }
+    assert(I != MBB.begin() && "Did not find target instruction while "
+                               "iterating backwards");
   }
 
   return std::make_pair(Survivor, Pos);