Regex: +isValid() with no parameter

[jankratochvil]

Summary

There would be some performance (only a little) improvement for RegularExpression::Execute.

[JDevlieghere]

LGTM

llvm/trunk/include/llvm/Support/Regex.h

Regex &operator=(Regex regex) {
       return *this;
     }
     Regex(Regex &&regex);
     ~Regex();
 
     /// isValid - returns the error encountered during regex compilation, or
     /// matching, if any.
     bool isValid(std::string &Error) const;
+    bool isValid() const { return !error; }
 
     /// getNumMatches - In a valid regex, return the number of parenthesized
     /// matches it contains.  The number filled in by match will include this
     /// many entries plus one for the whole regex (as element 0).
     unsigned getNumMatches() const;
 
     /// matches - Match the regex against a given \p String.
     ///