deps: cherry-pick 2e4da65 from v8's 4.8 upstream

Float v8 patch, which has been committed to v8 master and
backported to 4.8 and 4.9 in google repos, onto 4.8 v8 in
deps to resolve nodejs#5089

Original title/commit from google repos for 4.8 is:
 PPC: [turbofan] Support for CPU models lacking isel.
 v8/v8@2e4da65

deps/v8/src/compiler/ppc/code-generator-ppc.cc

@@ -1313,8 +1313,8 @@ void CodeGenerator::AssembleArchBoolean(Instruction* instr,
   PPCOperandConverter i(this, instr);
   Label done;
   ArchOpcode op = instr->arch_opcode();
-  bool check_unordered = (op == kPPC_CmpDouble);
   CRegister cr = cr0;
+  int reg_value = -1;
 
   // Overflow checked for add/sub only.
   DCHECK((condition != kOverflow && condition != kNotOverflow) ||
@@ -1326,44 +1326,44 @@ void CodeGenerator::AssembleArchBoolean(Instruction* instr,
   Register reg = i.OutputRegister(instr->OutputCount() - 1);
 
   Condition cond = FlagsConditionToCondition(condition);
-  switch (cond) {
-    case eq:
-    case lt:
+  if (op == kPPC_CmpDouble) {
+    // check for unordered if necessary
+    if (cond == le) {
+      reg_value = 0;
       __ li(reg, Operand::Zero());
-      __ li(kScratchReg, Operand(1));
-      __ isel(cond, reg, kScratchReg, reg, cr);
-      break;
-    case ne:
-    case ge:
+      __ bunordered(&done, cr);
+    } else if (cond == gt) {
+      reg_value = 1;
       __ li(reg, Operand(1));
-      __ isel(NegateCondition(cond), reg, r0, reg, cr);
-      break;
-    case gt:
-      if (check_unordered) {
-        __ li(reg, Operand(1));
+      __ bunordered(&done, cr);
+    }
+    // Unnecessary for eq/lt & ne/ge since only FU bit will be set.
+  }
+
+  if (CpuFeatures::IsSupported(ISELECT)) {
+    switch (cond) {
+      case eq:
+      case lt:
+      case gt:
+        if (reg_value != 1) __ li(reg, Operand(1));
         __ li(kScratchReg, Operand::Zero());
-        __ bunordered(&done, cr);
         __ isel(cond, reg, reg, kScratchReg, cr);
-      } else {
-        __ li(reg, Operand::Zero());
-        __ li(kScratchReg, Operand(1));
-        __ isel(cond, reg, kScratchReg, reg, cr);
-      }
-      break;
-    case le:
-      if (check_unordered) {
-        __ li(reg, Operand::Zero());
-        __ li(kScratchReg, Operand(1));
-        __ bunordered(&done, cr);
-        __ isel(NegateCondition(cond), reg, r0, kScratchReg, cr);
-      } else {
-        __ li(reg, Operand(1));
+        break;
+      case ne:
+      case ge:
+      case le:
+        if (reg_value != 1) __ li(reg, Operand(1));
+        // r0 implies logical zero in this form
         __ isel(NegateCondition(cond), reg, r0, reg, cr);
-      }
-      break;
+        break;
     default:
       UNREACHABLE();
       break;
+    }
+  } else {
+    if (reg_value != 0) __ li(reg, Operand::Zero());
+    __ b(NegateCondition(cond), &done, cr);
+    __ li(reg, Operand(1));
   }
   __ bind(&done);
 }