[ConstantRange] Rename a method and add more doc

Rename makeNoWrapRegion to a more obvious makeGuaranteedNoWrapRegion,
and add a comment about the counter-intuitive aspects of the function.
This is to help prevent cases like PR26628.

llvm-svn: 261532
diff --git a/llvm/lib/IR/ConstantRange.cpp b/llvm/lib/IR/ConstantRange.cpp
index 460b778..9bea98c 100644
--- a/llvm/lib/IR/ConstantRange.cpp
+++ b/llvm/lib/IR/ConstantRange.cpp
@@ -127,9 +127,9 @@
       .inverse();
 }
 
-ConstantRange ConstantRange::makeNoWrapRegion(Instruction::BinaryOps BinOp,
-                                              const APInt &C,
-                                              unsigned NoWrapKind) {
+ConstantRange
+ConstantRange::makeGuaranteedNoWrapRegion(Instruction::BinaryOps BinOp,
+                                          const APInt &C, unsigned NoWrapKind) {
   typedef OverflowingBinaryOperator OBO;
 
   // Computes the intersection of CR0 and CR1.  It is different from