[RegAlloc] Add additional tracing in InlineSpiller::rematerializeFor (llvm#160761)

We didn't have trace logging for two cases in this routine which makes
it sometimes hard to tell what is going on. In addition to debug trace
statements, add comments to explain the logic behind the early exits
which don't mark the virtual register live. Suggestions on how to word
these more precisely very welcome; I'm not clear I understand all the
intrinicies of this code myself.

Author: preames

llvm/lib/CodeGen/InlineSpiller.cpp

@@ -642,8 +642,12 @@ bool InlineSpiller::reMaterializeFor(LiveInterval &VirtReg, MachineInstr &MI) {
   SmallVector<std::pair<MachineInstr *, unsigned>, 8> Ops;
   VirtRegInfo RI = AnalyzeVirtRegInBundle(MI, VirtReg.reg(), &Ops);
 
-  if (!RI.Reads)
+  // Defs without reads will be deleted if unused after remat is
+  // completed for other users of the virtual register.
+  if (!RI.Reads) {
+    LLVM_DEBUG(dbgs() << "\tskipping remat of def " << MI);
     return false;
+  }
 
   SlotIndex UseIdx = LIS.getInstructionIndex(MI).getRegSlot(true);
   VNInfo *ParentVNI = VirtReg.getVNInfoAt(UseIdx.getBaseIndex());
@@ -657,8 +661,13 @@ bool InlineSpiller::reMaterializeFor(LiveInterval &VirtReg, MachineInstr &MI) {
     return true;
   }
 
-  if (SnippetCopies.count(&MI))
+  // Snippets copies are ignored for remat, and will be deleted if they
+  // don't feed a live user after rematerialization completes.
+  if (SnippetCopies.count(&MI)) {
+    LLVM_DEBUG(dbgs() << "\tskipping remat snippet copy for " << UseIdx << '\t'
+                      << MI);
     return false;
+  }
 
   LiveInterval &OrigLI = LIS.getInterval(Original);
   VNInfo *OrigVNI = OrigLI.getVNInfoAt(UseIdx);