Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -3966,7 +3966,10 @@
                                         unsigned Depth, AssumptionCache *AC,
                                         const Instruction *CxtI,
                                         const DominatorTree *DT) {
-  assert(LHS->getType() == RHS->getType() && "mismatched type");
+  // Types must match.
+  if (LHS->getType() != RHS->getType())
+    return None;
+
   Type *OpTy = LHS->getType();
   assert(OpTy->getScalarType()->isIntegerTy(1));
 
Index: lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1213,5 +1213,23 @@
     }
   }
 
+  // See if we can determine the result of this select based on a dominating
+  // condition.
+  BasicBlock *Parent = SI.getParent();
+  BasicBlock *Dom = Parent->getSinglePredecessor();
+  auto *PBI =
+      Dom ? dyn_cast_or_null<BranchInst>(Dom->getTerminator()) : nullptr;
+  if (PBI && PBI->isConditional() &&
+      PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
+      (PBI->getSuccessor(0) == Parent || PBI->getSuccessor(1) == Parent)) {
+    bool CondIsFalse = PBI->getSuccessor(1) == Parent;
+    Optional<bool> Implication = isImpliedCondition(
+        PBI->getCondition(), SI.getCondition(), DL, CondIsFalse);
+    if (Implication) {
+      Value *V = *Implication ? TrueVal : FalseVal;
+      return replaceInstUsesWith(SI, V);
+    }
+  }
+
   return nullptr;
 }
Index: test/Transforms/InstCombine/pr21210.ll
===================================================================
--- test/Transforms/InstCombine/pr21210.ll
+++ test/Transforms/InstCombine/pr21210.ll
@@ -40,7 +40,8 @@
   br label %b1
 
 b1:
-; CHECK: phi i32 [ %cond, %bb ], [ undef, %b0 ], [ %0, %entry ]
+; This seems perfectly fine to me?
+; CHECK: phi i32 [ %len, %bb ], [ undef, %b0 ], [ %0, %entry ]
   %1 = phi i32 [ %cond, %bb ], [ undef, %b0 ], [ %0, %entry ]
   br label %ret
 
Index: test/Transforms/InstCombine/select-implied.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/select-implied.ll
@@ -0,0 +1,54 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; A == B implies A >u B is false.
+; CHECK-LABEL: @test1
+; CHECK: call void @foo(i32 10)
+define void @test1(i32 %a, i32 %b) {
+  %cmp1 = icmp eq i32 %a, %b
+  br i1 %cmp1, label %taken, label %end
+
+taken:
+  %cmp2 = icmp ugt i32 %a, %b
+  %c = select i1 %cmp2, i32 0, i32 10
+  call void @foo(i32 %c)
+  br label %end
+
+end:
+  ret void
+}
+
+; If A == B is false then A != B is true.
+; CHECK-LABEL: @test2
+; CHECK: call void @foo(i32 20)
+define void @test2(i32 %a, i32 %b) {
+  %cmp1 = icmp eq i32 %a, %b
+  br i1 %cmp1, label %end, label %taken
+
+taken:
+  %cmp2 = icmp ne i32 %a, %b
+  %c = select i1 %cmp2, i32 20, i32 0
+  call void @foo(i32 %c)
+  br label %end
+
+end:
+  ret void
+}
+
+; A >u 10 implies A >u 10 is true.
+; CHECK-LABEL: @test3
+; CHECK: call void @foo(i32 30)
+define void @test3(i32 %a, i32 %b) {
+  %cmp1 = icmp ugt i32 %a, 10
+  br i1 %cmp1, label %taken, label %end
+
+taken:
+  %cmp2 = icmp ugt i32 %a, 10
+  %c = select i1 %cmp2, i32 30, i32 0
+  call void @foo(i32 %c)
+  br label %end
+
+end:
+  ret void
+}
+
+declare void @foo(i32)