[ConstantRange] Add assertion for KnownBits validity; NFC

Following the suggestion in D59475.

llvm-svn: 356346

llvm/lib/IR/ConstantRange.cpp

@@ -56,6 +56,8 @@ ConstantRange::ConstantRange(APInt L, APInt U)
 
 ConstantRange ConstantRange::fromKnownBits(const KnownBits &Known,
                                            bool IsSigned) {
+  assert(!Known.hasConflict() && "Expected valid KnownBits");
+
   if (Known.isUnknown())
     return ConstantRange(Known.getBitWidth(), /* full */ true);