[SIL] Phi with incoming lexical value is lexical.

Added SILPhiArgument::isLexical.  It's true for phis which have an
incoming value that's lexical.  Dispatch to this member from
ValueBase::isLexical.

include/swift/SIL/SILArgument.h

@@ -241,6 +241,9 @@ public:
   /// result.
   bool isPhi() const;
 
+  /// Whether any of the values incoming to this phi are lexical.
+  bool isLexical() const;
+
   /// Return true if this block argument is a terminator result.
   bool isTerminatorResult() const { return !isPhi(); }