PPC: [turbofan] Optimize loading 64-bit SMIs.

Especially when loading and untagging SMIs within code stubs,
instances of the following pattern appear in the generated code:

    ld    <dst>, <offset>(<base>)
    sradi <dst>,<dst>,32

This CL changes that code to:

    lwa   <dst>, <SmiWordOffset(offset)>(<base>)

R=joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1855533002

Cr-Commit-Position: refs/heads/master@{#35216}

src/compiler/ppc/instruction-selector-ppc.cc

@@ -863,6 +863,30 @@ void InstructionSelector::VisitWord32PairSar(Node* node) {
 
 #if V8_TARGET_ARCH_PPC64
 void InstructionSelector::VisitWord64Sar(Node* node) {
+  PPCOperandGenerator g(this);
+  Int64BinopMatcher m(node);
+  if (CanCover(m.node(), m.left().node()) && m.left().IsLoad() &&
+      m.right().Is(32)) {
+    // Just load and sign-extend the interesting 4 bytes instead. This happens,
+    // for example, when we're loading and untagging SMIs.
+    BaseWithIndexAndDisplacement64Matcher mleft(m.left().node(), true);
+    if (mleft.matches() && mleft.index() == nullptr) {
+      int64_t offset = 0;
+      Node* displacement = mleft.displacement();
+      if (displacement != nullptr) {
+        Int64Matcher mdisplacement(displacement);
+        DCHECK(mdisplacement.HasValue());
+        offset = mdisplacement.Value();
+      }
+      offset = SmiWordOffset(offset);
+      if (g.CanBeImmediate(offset, kInt16Imm_4ByteAligned)) {
+        Emit(kPPC_LoadWordS32 | AddressingModeField::encode(kMode_MRI),
+             g.DefineAsRegister(node), g.UseRegister(mleft.base()),
+             g.TempImmediate(offset));
+        return;
+      }
+    }
+  }
   VisitRRO(this, kPPC_ShiftRightAlg64, node, kShift64Imm);
 }
 #endif