[turbofan] Make --branch-load-poisoning work with stock snapshot.

This makes sure that an existing unmodified snapshot works together with
the --branch-load-poisoning feature by unconditionally resetting the
underlying register in {CEntryStub::Generate}.

R=mvstanton@chromium.org

Change-Id: I4d9f12a3b45bd7b46eda55a342beadf9b9a4bb8a
Reviewed-on: https://chromium-review.googlesource.com/926365
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51397}

src/arm/code-stubs-arm.cc

@@ -385,10 +385,11 @@ void CEntryStub::Generate(MacroAssembler* masm) {
   __ cmp(cp, Operand(0));
   __ str(cp, MemOperand(fp, StandardFrameConstants::kContextOffset), ne);
 
-  // Reset the masking register.
-  if (FLAG_branch_load_poisoning) {
+  // Reset the masking register. This is done independent of the underlying
+  // feature flag {FLAG_branch_load_poisoning} to make the snapshot work with
+  // both configurations. It is safe to always do this, because the underlying
+  // register is caller-saved and can be arbitrarily clobbered.
   __ ResetSpeculationPoisonRegister();
-  }
 
   // Compute the handler entry address and jump to it.
   ConstantPoolUnavailableScope constant_pool_unavailable(masm);

src/x64/code-stubs-x64.cc

@@ -425,10 +425,11 @@ void CEntryStub::Generate(MacroAssembler* masm) {
   __ movp(Operand(rbp, StandardFrameConstants::kContextOffset), rsi);
   __ bind(&skip);
 
-  // Reset the masking register.
-  if (FLAG_branch_load_poisoning) {
+  // Reset the masking register. This is done independent of the underlying
+  // feature flag {FLAG_branch_load_poisoning} to make the snapshot work with
+  // both configurations. It is safe to always do this, because the underlying
+  // register is caller-saved and can be arbitrarily clobbered.
   __ ResetSpeculationPoisonRegister();
-  }
 
   // Compute the handler entry address and jump to it.
   __ movp(rdi, masm->ExternalOperand(pending_handler_entrypoint_address));