[ConstantRange] Add getNonEmpty() constructor

ConstantRanges have an annoying special case: If upper and lower are
the same, it can be either an empty or a full set. When constructing
constant ranges nearly always a full set is intended, but this still
requires an explicit check in many places.

This revision adds a getNonEmpty() constructor that disambiguates this
case: If upper and lower are the same, a full set is created.

Differential Revision: https://reviews.llvm.org/D60947

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358854 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ValueTracking.cpp

@@ -5726,8 +5726,7 @@ ConstantRange llvm::computeConstantRange(const Value *V, bool UseInstrInfo) {
   else if (auto *SI = dyn_cast<SelectInst>(V))
     setLimitsForSelectPattern(*SI, Lower, Upper);
 
-  ConstantRange CR = Lower != Upper ? ConstantRange(Lower, Upper)
-                                    : ConstantRange::getFull(BitWidth);
+  ConstantRange CR = ConstantRange::getNonEmpty(Lower, Upper);
 
   if (auto *I = dyn_cast<Instruction>(V))
     if (auto *Range = IIQ.getMetadata(I, LLVMContext::MD_range))