Fix FlooringDivByPowerOf2I on ARM/ARM64

Fix for divisor=1, found during code inspection. We can't hit this bug, due to
HDiv::Canonicalize() discarding the operation, but it might avoid a future bug
hunt. Patch also includes a small tidying of the ARM64 code.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/280883003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21313 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/arm/lithium-codegen-arm.cc

@@ -1435,9 +1435,14 @@ void LCodeGen::DoFlooringDivByPowerOf2I(LFlooringDivByPowerOf2I* instr) {
   Register result = ToRegister(instr->result());
   int32_t divisor = instr->divisor();
 
+  // If the divisor is 1, return the dividend.
+  if (divisor == 1) {
+    __ Move(result, dividend);
+    return;
+  }
+
   // If the divisor is positive, things are easy: There can be no deopts and we
   // can simply do an arithmetic right shift.
-  if (divisor == 1) return;
   int32_t shift = WhichPowerOf2Abs(divisor);
   if (divisor > 1) {
     __ mov(result, Operand(dividend, ASR, shift));

src/arm64/lithium-codegen-arm64.cc

@@ -3879,9 +3879,14 @@ void LCodeGen::DoFlooringDivByPowerOf2I(LFlooringDivByPowerOf2I* instr) {
   Register result = ToRegister32(instr->result());
   int32_t divisor = instr->divisor();
 
+  // If the divisor is 1, return the dividend.
+  if (divisor == 1) {
+    __ Mov(result, dividend, kDiscardForSameWReg);
+    return;
+  }
+
   // If the divisor is positive, things are easy: There can be no deopts and we
   // can simply do an arithmetic right shift.
-  if (divisor == 1) return;
   int32_t shift = WhichPowerOf2Abs(divisor);
   if (divisor > 1) {
     __ Mov(result, Operand(dividend, ASR, shift));
@@ -3906,14 +3911,8 @@ void LCodeGen::DoFlooringDivByPowerOf2I(LFlooringDivByPowerOf2I* instr) {
     return;
   }
 
-  // Using a conditional data processing instruction would need 1 more register.
-  Label not_kmin_int, done;
-  __ B(vc, &not_kmin_int);
-  __ Mov(result, kMinInt / divisor);
-  __ B(&done);
-  __ bind(&not_kmin_int);
-  __ Mov(result, Operand(dividend, ASR, shift));
-  __ bind(&done);
+  __ Asr(result, dividend, shift);
+  __ Csel(result, result, kMinInt / divisor, vc);
 }