[SimplifyCFG] Fix value enumeration of a full range (#166379)

ConstantRange uses `[-1, -1)` as the canonical form of a full set.
Therefore, the `for (APInt I = Lower; I != Upper; ++I)` idiom doesn't
work for full ranges. This patch fixes the value enumeration in
`ConstantComparesGatherer` to prevent missing values for full sets.

Closes #166369.

Author: dtcxzyw

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -778,8 +778,10 @@ struct ConstantComparesGatherer {
       return false;
 
     // Add all values from the range to the set
-    for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp)
+    APInt Tmp = Span.getLower();
+    do
       Vals.push_back(ConstantInt::get(I->getContext(), Tmp));
+    while (++Tmp != Span.getUpper());
 
     UsedICmps++;
     return true;

llvm/test/Transforms/SimplifyCFG/pr166369.ll

@@ -0,0 +1,37 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -S -passes=simplifycfg < %s | FileCheck %s
+
+; Make sure we handle full-set ranges correctly.
+define void @test_i1() {
+; CHECK-LABEL: define void @test_i1() {
+; CHECK-NEXT:  [[BB:.*:]]
+; CHECK-NEXT:    ret void
+;
+bb:
+  %icmp = icmp ugt i1 false, true
+  br label %bb5
+
+bb5:
+  %select = select i1 %icmp, i1 %icmp, i1 false
+  br i1 %select, label %bb5, label %bb6
+
+bb6:
+  ret void
+}
+
+define void @test_i3() {
+; CHECK-LABEL: define void @test_i3() {
+; CHECK-NEXT:  [[BB:.*:]]
+; CHECK-NEXT:    ret void
+;
+bb:
+  %icmp = icmp ugt i3 0, 7
+  br label %bb5
+
+bb5:
+  %select = select i1 %icmp, i1 %icmp, i1 false
+  br i1 %select, label %bb5, label %bb6
+
+bb6:
+  ret void
+}