[Analysis] recognize the 'null' pointer constant as not poison

[spatel]

Summary

Just making sure that I'm not missing some corner-case for pointers here...
This is needed to create a safe-constant-substitution alternate to D78430.

[aqjune]

LGTM

llvm/lib/Analysis/ValueTracking.cpp

bool llvm::isGuaranteedNotToBeUndefOrPoison(const Value *V,
   // nor poison if their arguments are not poison/undef.
 
   if (auto *C = dyn_cast<Constant>(V)) {
     // TODO: We can analyze ConstExpr by opcode to determine if there is any
     //       possibility of poison.
     if (isa<UndefValue>(C) || isa<ConstantExpr>(C))
       return false;
 
-    // TODO: Add ConstantFP and pointers.
-    if (isa<ConstantInt>(C) || isa<GlobalVariable>(C) )
+    // TODO: Add ConstantFP.
+    if (isa<ConstantInt>(C) || isa<GlobalVariable>(C) ||
+        isa<ConstantPointerNull>(C))
       return true;
 
     if (C->getType()->isVectorTy())
       return !C->containsUndefElement() && !C->containsConstantExpression();
 
     // TODO: Recursively analyze aggregates or other constants.
     return false;
   }

llvm/test/Transforms/InstSimplify/freeze.ll

 ; CHECK-LABEL: @make_const_fn(
 ; CHECK-NEXT:    [[K:%.*]] = freeze i32 ()* @make_const
 ; CHECK-NEXT:    ret i32 ()* [[K]]
 ;
   %k = freeze i32()* @make_const
   ret i32()* %k
 }
 
-; TODO: This is not poison.
-
 define i32* @make_const_null() {
 ; CHECK-LABEL: @make_const_null(
-; CHECK-NEXT:    [[K:%.*]] = freeze i32* null
-; CHECK-NEXT:    ret i32* [[K]]
+; CHECK-NEXT:    ret i32* null
 ;
   %k = freeze i32* null
   ret i32* %k
 }
 
 define <2 x i32> @constvector() {
 ; CHECK-LABEL: @constvector(
 ; CHECK-NEXT:    ret <2 x i32> <i32 0, i32 1>
;
   %p = alloca [4 x i8]
   %q = getelementptr inbounds [4 x i8], [4 x i8]* %p, i32 0, i32 %arg
   %q2 = freeze i8* %q
   ret i8* %q2
 }
 
 define i32* @gep_inbounds_null() {
 ; CHECK-LABEL: @gep_inbounds_null(
-; CHECK-NEXT:    [[K:%.*]] = freeze i32* null
-; CHECK-NEXT:    ret i32* [[K]]
+; CHECK-NEXT:    ret i32* null
 ;
   %p = getelementptr inbounds i32, i32* null, i32 0
   %k = freeze i32* %p
   ret i32* %k
 }
 
 define i32* @gep_inbounds_null_noopt(i32* %p) {
 ; CHECK-LABEL: @gep_inbounds_null_noopt(