Revert "[regalloc] More aggressively reuse spill ranges for phi inputs"

Turns out TryReuseSpillForPhi does more than reusing a spill slot for a
phi (which is beneficial in general). It also decides whether a phi
should start out to be allocated in a spill slot. The latter, of course,
benefits from control flow knowledge. Hence, this change was detrimental
in cases where a common input to a phi is only spilled on few control
flow pathes.

To fix, we need to disentangle spill-slot reuse and the decision whether
a phi should start out spilled. I will look into this in a follow up
change.

This reverts commit b79cbd56.

Change-Id: I228185bb1a4b320d3115ba7f1d921593480d8e7d
Reviewed-on: https://chromium-review.googlesource.com/c/1304549Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57113}

src/compiler/register-allocator.cc

@@ -3355,6 +3355,7 @@ bool LinearScanAllocator::TryReuseSpillForPhi(TopLevelLiveRange* range) {
   RegisterAllocationData::PhiMapValue* phi_map_value =
       data()->GetPhiMapValueFor(range);
   const PhiInstruction* phi = phi_map_value->phi();
+  const InstructionBlock* block = phi_map_value->block();
   // Count the number of spilled operands.
   size_t spilled_count = 0;
   LiveRange* first_op = nullptr;
@@ -3362,9 +3363,15 @@ bool LinearScanAllocator::TryReuseSpillForPhi(TopLevelLiveRange* range) {
     int op = phi->operands()[i];
     LiveRange* op_range = data()->GetOrCreateLiveRangeFor(op);
     if (!op_range->TopLevel()->HasSpillRange()) continue;
-    if (!op_range->TopLevel()->IsSpilledOnlyInDeferredBlocks()) {
-      // This value was spilled at definition, so it is available in a spillslot
-      // regardless of control flow.
+    const InstructionBlock* pred =
+        code()->InstructionBlockAt(block->predecessors()[i]);
+    LifetimePosition pred_end =
+        LifetimePosition::InstructionFromInstructionIndex(
+            pred->last_instruction_index());
+    while (op_range != nullptr && !op_range->CanCover(pred_end)) {
+      op_range = op_range->next();
+    }
+    if (op_range != nullptr && op_range->spilled()) {
       spilled_count++;
       if (first_op == nullptr) {
         first_op = op_range->TopLevel();