[InlineAdvisor] Add option to control deferred inlining (NFC)

This change is split out from D115497 to add the option
independently from the switch of the default value.

Author: nikic

llvm/include/llvm/Analysis/InlineCost.h

@@ -212,7 +212,7 @@ struct InlineParams {
   Optional<bool> ComputeFullInlineCost;
 
   /// Indicate whether we should allow inline deferral.
-  Optional<bool> EnableDeferral = true;
+  Optional<bool> EnableDeferral;
 };
 
 /// Generate the parameters to tune the inline cost analysis based only on the

llvm/lib/Analysis/InlineAdvisor.cpp

@@ -40,6 +40,10 @@ static cl::opt<bool>
                                    " callsites processed by inliner but decided"
                                    " to be not inlined"));
 
+static cl::opt<bool> EnableInlineDeferral("inline-deferral", cl::init(true),
+                                          cl::Hidden,
+                                          cl::desc("Enable deferred inlining"));
+
 // An integer used to limit the cost of inline deferral.  The default negative
 // number tells shouldBeDeferred to only take the secondary cost into account.
 static cl::opt<int>
@@ -100,8 +104,9 @@ llvm::Optional<llvm::InlineCost> static getDefaultInlineAdvice(
     return getInlineCost(CB, Params, CalleeTTI, GetAssumptionCache, GetTLI,
                          GetBFI, PSI, RemarksEnabled ? &ORE : nullptr);
   };
-  return llvm::shouldInline(CB, GetInlineCost, ORE,
-                            Params.EnableDeferral.getValueOr(false));
+  return llvm::shouldInline(
+      CB, GetInlineCost, ORE,
+      Params.EnableDeferral.getValueOr(EnableInlineDeferral));
 }
 
 std::unique_ptr<InlineAdvice>