[ValueTracking] Handle non-zero ashr/lshr recurrences

If we know we don't shift out bits (e.g. exact), all we need to know is that input is non-zero.

Author: preames

llvm/lib/Analysis/ValueTracking.cpp

@@ -2237,6 +2237,9 @@ static bool isNonZeroRecurrence(const PHINode *PN) {
   case Instruction::Shl:
     return !StartC->isNullValue() &&
            (BO->hasNoUnsignedWrap() || BO->hasNoSignedWrap());
+  case Instruction::AShr:
+  case Instruction::LShr:
+    return !StartC->isNullValue() && BO->isExact();
   default:
     return false;
   }

llvm/test/Analysis/ValueTracking/monotonic-phi.ll

@@ -486,8 +486,7 @@ define i1 @test_lshr_exact(i8 %p, i8* %pq, i8 %n) {
 ; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i8 [[A]], [[N:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP1]], label [[EXIT:%.*]], label [[LOOP]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[A]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 false
 ;
 entry:
   br label %loop
@@ -561,8 +560,7 @@ define i1 @test_ashr_exact(i8 %p, i8* %pq, i8 %n) {
 ; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i8 [[A]], [[N:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP1]], label [[EXIT:%.*]], label [[LOOP]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[A]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 false
 ;
 entry:
   br label %loop