[FaultMaps] Let the frontend pre-select implicit null check candidates.

Summary:
This change introduces a !make.implicit metadata that allows the
frontend to pre-select the set of explicit null checks that will be
considered for transformation into implicit null checks.

The reason for not using profiling data instead of !make.implicit is
explained in the change to `FaultMaps.rst`.

Reviewers: atrick, reames, pgavlin, JosephTremoulet

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10824

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241116 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ImplicitNullChecks.cpp

@@ -124,6 +124,13 @@ bool ImplicitNullChecks::analyzeBlockForNullChecks(
     MachineBasicBlock &MBB, SmallVectorImpl<NullCheck> &NullCheckList) {
   typedef TargetInstrInfo::MachineBranchPredicate MachineBranchPredicate;
 
+  MDNode *BranchMD =
+      MBB.getBasicBlock()
+          ? MBB.getBasicBlock()->getTerminator()->getMetadata("make.implicit")
+          : nullptr;
+  if (!BranchMD)
+    return false;
+
   MachineBranchPredicate MBP;
 
   if (TII->AnalyzeBranchPredicate(MBB, MBP, true))