Reapply r237520 with another fix for infinite looping

SimplifyDemandedBits was "simplifying" a constant by removing just sign bits.
This caused a canonicalization race between different parts of instcombine.

Fix and regression test added - third time lucky?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237539 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "InstCombineInternal.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/PatternMatch.h"
 
@@ -406,6 +407,12 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
     break;
   }
   case Instruction::Select:
+    // If this is a select as part of a min/max pattern, don't simplify any
+    // further in case we break the structure.
+    Value *LHS, *RHS;
+    if (matchSelectPattern(I, LHS, RHS) != SPF_UNKNOWN)
+      return nullptr;
+      
     if (SimplifyDemandedBits(I->getOperandUse(2), DemandedMask, RHSKnownZero,
                              RHSKnownOne, Depth + 1) ||
         SimplifyDemandedBits(I->getOperandUse(1), DemandedMask, LHSKnownZero,