[STLExtras] Add size() for ranges, and remove distance()

r332057 introduced distance() for ranges. Based on post-commit feedback,
this renames distance() to size(). The new size() is also only enabled
when the operation is O(1).

Differential Revision: https://reviews.llvm.org/D46976

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332551 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ImplicitNullChecks.cpp

@@ -597,7 +597,7 @@ MachineInstr *ImplicitNullChecks::insertFaultingInstr(
   unsigned DefReg = NoRegister;
   if (NumDefs != 0) {
     DefReg = MI->defs().begin()->getReg();
-    assert(distance(MI->defs()) == 1 && "expected exactly one def!");
+    assert(NumDefs == 1 && "expected exactly one def!");
   }
 
   FaultMaps::FaultKind FK;