PERF: RangeIndex.get_loc
Small simplification gives a small speedup

```
In [2]: rng = pd.Index(range(10**5)) 

In [3]: %timeit rng.get_loc(5.0)                                                
6.38 µs ± 381 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)  <-- master
1.15 µs ± 40.9 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)  <-- PR

In [5]: %timeit rng.get_loc(5)                                                  
845 ns ± 5.71 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)  <--master
In [5]: %timeit rng.get_loc(5)                                                                                                                                                                            
860 ns ± 16.7 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)  <-- PR

In [9]: def foo(): 
   ...:     try: 
   ...:         return rng.get_loc(None) 
   ...:     except KeyError: 
   ...:         pass 
   ...:                                                                         

In [10]: %timeit foo()                                                          
6.72 µs ± 656 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)  <-- master
1.11 µs ± 94 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)  <-- PR
```