Restructure some assertion checking based on post commit feedback by Aaron and Tom.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223150 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Statepoint.h

@@ -129,7 +129,13 @@ class StatepointBase {
 
 
 #ifndef NDEBUG
+  /// Asserts if this statepoint is malformed.  Common cases for failure
+  /// include incorrect length prefixes for variable length sections or
+  /// illegal values for parameters.
   void verify() {
+    assert(numCallArgs() >= 0 &&
+           "number of arguments to actually callee can't be negative");
+
     // The internal asserts in the iterator accessors do the rest.
     (void)call_args_begin();
     (void)call_args_end();