PPC: Bugfix: assert in lithium compile for LMaybeGrowElements

Port ce1fe78d

R=mvstanton@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, bjaideep@ca.ibm.com
BUG=chromium:585041
LOG=N

Review URL: https://codereview.chromium.org/1872443002

Cr-Commit-Position: refs/heads/master@{#35337}

src/crankshaft/ppc/lithium-codegen-ppc.cc

@@ -4429,7 +4429,15 @@ void LCodeGen::DoDeferredMaybeGrowElements(LMaybeGrowElements* instr) {
 
     LOperand* key = instr->key();
     if (key->IsConstantOperand()) {
-      __ LoadSmiLiteral(r6, ToSmi(LConstantOperand::cast(key)));
+      LConstantOperand* constant_key = LConstantOperand::cast(key);
+      int32_t int_key = ToInteger32(constant_key);
+      if (Smi::IsValid(int_key)) {
+        __ LoadSmiLiteral(r6, Smi::FromInt(int_key));
+      } else {
+        // We should never get here at runtime because there is a smi check on
+        // the key before this point.
+        __ stop("expected smi");
+      }
     } else {
       __ SmiTag(r6, ToRegister(key));
     }