[ConstantRange] Implement getSignedMin/Max in a less complicated and faster way

[craig.topper]

Summary

As far as I can tell we should be able to implement these almost the same way we do unsigned, but using signed comparisons and checks for min signed value instead of min unsigned value.

[craig.topper]

Ping

[davide]

LGTM. THanks for doing this Craig.

llvm/trunk/lib/IR/ConstantRange.cpp

 
 APInt ConstantRange::getUnsignedMin() const {
   if (isFullSet() || (isWrappedSet() && !getUpper().isNullValue()))
     return APInt::getMinValue(getBitWidth());
   return getLower();
 }
 
 APInt ConstantRange::getSignedMax() const {
-  if (!isWrappedSet()) {
-    APInt UpperMinusOne = getUpper() - 1;
-    if (getLower().sle(UpperMinusOne))
-      return UpperMinusOne;
-    return APInt::getSignedMaxValue(getBitWidth());
-  }
-  if (getLower().isNegative() == getUpper().isNegative())
+  if (isFullSet() || Lower.sgt(Upper))
     return APInt::getSignedMaxValue(getBitWidth());
   return getUpper() - 1;
 }
 
 APInt ConstantRange::getSignedMin() const {
-  if (!isWrappedSet()) {
-    if (getLower().sle(getUpper() - 1))
-      return getLower();
-    return APInt::getSignedMinValue(getBitWidth());
-  }
-  if ((getUpper() - 1).slt(getLower())) {
-    if (!getUpper().isMinSignedValue())
-      return APInt::getSignedMinValue(getBitWidth());
-  }
+  if (isFullSet() || (Lower.sgt(Upper) && !getUpper().isMinSignedValue()))
   return getLower();
 }
 
 bool ConstantRange::contains(const APInt &V) const {
   if (Lower == Upper)
     return isFullSet();
 
   if (!isWrappedSet())