Relax an assert a bit to avoid a crash on unreachable code.

Patch by Duncan Exon Smith with a small tweak by me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222984 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/MemoryDependenceAnalysis.cpp

@@ -1361,7 +1361,7 @@ getNonLocalPointerDepFromBB(const PHITransAddr &Pointer,
       if (I->getBB() != BB)
         continue;
 
-      assert(I->getResult().isNonLocal() &&
+      assert((I->getResult().isNonLocal() || !DT->isReachableFromEntry(BB)) &&
              "Should only be here with transparent block");
       I->setResult(MemDepResult::getUnknown());
       Result.push_back(NonLocalDepResult(I->getBB(), I->getResult(),

test/Transforms/GVN/load-from-unreachable-predecessor.ll

@@ -0,0 +1,20 @@
+; RUN: opt -gvn -S < %s | FileCheck %s
+
+; Check that an unreachable predecessor to a PHI node doesn't cause a crash.
+; PR21625.
+
+define i32 @f(i32** %f) {
+; CHECK: bb0:
+; Load should be removed, since it's ignored.
+; CHECK-NEXT: br label
+bb0:
+  %bar = load i32** %f
+  br label %bb2
+bb1:
+  %zed = load i32** %f
+  br i1 false, label %bb1, label %bb2
+bb2:
+  %foo = phi i32* [ null, %bb0 ], [ %zed, %bb1 ]
+  %storemerge = load i32* %foo
+  ret i32 %storemerge
+}