[RegAlloc] Strengthen asserts in LiveRangeEdit::scanRemattable [nfc] (llvm#160765)

We should always be able to find the VNInfo in the original live
interval which corresponds to the subset we're trying to spill, and the
only cases where we have a VNInfo without a definition instruction are
if the vni is unused, or corresponds to a phi. Adjust the code structure
to explicitly check for PHIDef, and assert the stronger conditions.

Author: preames

llvm/lib/CodeGen/LiveRangeEdit.cpp

@@ -75,11 +75,11 @@ void LiveRangeEdit::scanRemattable() {
     Register Original = VRM->getOriginal(getReg());
     LiveInterval &OrigLI = LIS.getInterval(Original);
     VNInfo *OrigVNI = OrigLI.getVNInfoAt(VNI->def);
-    if (!OrigVNI)
+    assert(OrigVNI && "Corrupt interval mapping?");
+    if (OrigVNI->isPHIDef())
       continue;
     MachineInstr *DefMI = LIS.getInstructionFromIndex(OrigVNI->def);
-    if (!DefMI)
-      continue;
+    assert(DefMI && "Missing instruction for def slot");
     if (TII.isReMaterializable(*DefMI))
       Remattable.insert(OrigVNI);
   }