[DebugInfo] Remove LexicalScopes::isCurrentFunctionScope and CSE a use of LexicalScopes::getCurrentFunctionScope

Now that we're sure the only root (non-abstract) scope is the current
function scope, there's no need for isCurrentFunctionScope, the property
can be tested directly instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220451 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LexicalScopes.h

@@ -148,12 +148,6 @@ public:
   /// empty - Return true if there is any lexical scope information available.
   bool empty() { return CurrentFnLexicalScope == nullptr; }
 
-  /// isCurrentFunctionScope - Return true if given lexical scope represents
-  /// current function.
-  bool isCurrentFunctionScope(const LexicalScope *LS) {
-    return LS == CurrentFnLexicalScope;
-  }
-
   /// getCurrentFunctionScope - Return lexical scope for the current function.
   LexicalScope *getCurrentFunctionScope() const {
     return CurrentFnLexicalScope;