[SCEV] accurate range for addrecexpr with nuw flag

If addrecexpr has nuw flag, the value should never be less than its
start value and start value does not required to be SCEVConstant.

Reviewed By: nikic, sanjoy

Differential Revision: https://reviews.llvm.org/D71690

Author: Zheng Chen

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -5672,11 +5672,12 @@ ScalarEvolution::getRangeRef(const SCEV *S,
   if (const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(S)) {
     // If there's no unsigned wrap, the value will never be less than its
     // initial value.
-    if (AddRec->hasNoUnsignedWrap())
-      if (const SCEVConstant *C = dyn_cast<SCEVConstant>(AddRec->getStart()))
-        if (!C->getValue()->isZero())
-          ConservativeResult = ConservativeResult.intersectWith(
-              ConstantRange(C->getAPInt(), APInt(BitWidth, 0)), RangeType);
+    if (AddRec->hasNoUnsignedWrap()) {
+      APInt UnsignedMinValue = getUnsignedRangeMin(AddRec->getStart());
+      if (!UnsignedMinValue.isNullValue())
+        ConservativeResult = ConservativeResult.intersectWith(
+            ConstantRange(UnsignedMinValue, APInt(BitWidth, 0)), RangeType);
+    }
 
     // If there's no signed wrap, and all the operands except initial value have
     // the same sign or zero, the value won't ever be:

llvm/test/Analysis/ScalarEvolution/range_nw_flag.ll

@@ -2,7 +2,7 @@
 
 ; copied from flags-from-poison.ll
 ; CHECK-LABEL: @test-add-nuw
-; CHECK: -->  {(1 + %offset)<nuw>,+,1}<nuw><%loop> U: full-set S: full-set
+; CHECK: -->  {(1 + %offset)<nuw>,+,1}<nuw><%loop> U: [1,0) S: [1,0)
 define void @test-add-nuw(float* %input, i32 %offset, i32 %numIterations) {
 entry:
   br label %loop
@@ -20,7 +20,7 @@ exit:
 }
 
 ; CHECK-LABEL: @test-addrec-nuw
-; CHECK: -->  {(1 + (10 smax %offset))<nuw>,+,1}<nuw><%loop> U: full-set S: full-set 
+; CHECK: -->  {(1 + (10 smax %offset))<nuw>,+,1}<nuw><%loop> U: [11,0) S: [11,0)
 define void @test-addrec-nuw(float* %input, i32 %offset, i32 %numIterations) {
 entry:
   %cmp = icmp sgt i32 %offset, 10