[SCEV] Don't require positive BTC when non-zero is sufficient

The only thing we care about here is that we don't exit on the
first iteration. Whether the BTC is large enough to overflow the
signed integer space is not relevant.

Author: nikic

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -9891,7 +9891,7 @@ const SCEV *ScalarEvolution::computeSCEVAtScope(const SCEV *V, const Loop *L) {
         // Do we have a loop invariant value flowing around the backedge
         // for a loop which must execute the backedge?
         if (!isa<SCEVCouldNotCompute>(BackedgeTakenCount) &&
-            isKnownPositive(BackedgeTakenCount) &&
+            isKnownNonZero(BackedgeTakenCount) &&
             PN->getNumIncomingValues() == 2) {
 
           unsigned InLoopPred =

llvm/test/Transforms/IndVarSimplify/exit_value_tests.ll

@@ -172,16 +172,7 @@ loopexit:
 define i32 @unroll_phi_select_constant_nonzero_large_btc(i32 %arg1, i32 %arg2) {
 ; CHECK-LABEL: @unroll_phi_select_constant_nonzero_large_btc(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[I_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[SELECTOR:%.*]] = phi i32 [ [[ARG1:%.*]], [[ENTRY]] ], [ [[ARG2:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[I_NEXT]] = add nuw i32 [[I]], 1
-; CHECK-NEXT:    [[C:%.*]] = icmp ult i32 [[I]], -42
-; CHECK-NEXT:    br i1 [[C]], label [[LOOP]], label [[LOOPEXIT:%.*]]
-; CHECK:       loopexit:
-; CHECK-NEXT:    [[SELECTOR_LCSSA:%.*]] = phi i32 [ [[SELECTOR]], [[LOOP]] ]
-; CHECK-NEXT:    ret i32 [[SELECTOR_LCSSA]]
+; CHECK-NEXT:    ret i32 [[ARG2:%.*]]
 ;
 entry:
   br label %loop