[nci] Extract feedback collection experiment to dedicated flag

This adds a dedicated --turbo-collect-feedback-in-generic-lowering
flag instead of piggy-backing on top of --turbo-nci in order to free
that up for upcoming work.

The new flag is temporary and can be removed once we've collected
enough data and made a decision on whether to enable it
unconditionally.

Bug: v8:8888
Change-Id: I5c0fd35e46b4c0237c266ba6253b9c5cb4cd7995
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2237137
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68287}

src/compiler/globals.h

@@ -20,7 +20,9 @@ namespace compiler {
 //
 // TODO(jgruber): Remove once we've made a decision whether to collect feedback
 // unconditionally.
-inline bool CollectFeedbackInGenericLowering() { return FLAG_turbo_nci; }
+inline bool CollectFeedbackInGenericLowering() {
+  return FLAG_turbo_collect_feedback_in_generic_lowering;
+}
 
 enum class StackCheckKind {
   kJSFunctionEntry = 0,

src/flags/flag-definitions.h

@@ -663,6 +663,8 @@ DEFINE_BOOL(turbo_nci_as_highest_tier, false,
             "purposes.")
 DEFINE_BOOL(print_nci_code, false, "print native context independent code.")
 DEFINE_BOOL(trace_turbo_nci, false, "trace native context independent code.")
+DEFINE_BOOL(turbo_collect_feedback_in_generic_lowering, false,
+            "enable experimental feedback collection in generic lowering.")
 
 // Favor memory over execution speed.
 DEFINE_BOOL(optimize_for_size, false,