[Doc] Fix LangRef links referred in GetElementPtr.rst

[• dberlin]

This is obvious, please feel free to commit things like this in the future without review.

[chenwj]

In D32929#760181, @dberlin wrote:

This is obvious, please feel free to commit things like this in the future without review.

@dberlin, I have not commit access, so would you help me to land it? :-)

llvm/trunk/docs/GetElementPtr.rst

 =======================================
 The Often Misunderstood GEP Instruction
 =======================================
 
 .. contents::
    :local:
 
 Introduction
 ============
 
 This document seeks to dispel the mystery and confusion surrounding LLVM's
-`GetElementPtr <LangRef.html#i_getelementptr>`_ (GEP) instruction.  Questions
-about the wily GEP instruction are probably the most frequently occurring
-questions once a developer gets down to coding with LLVM. Here we lay out the
-sources of confusion and show that the GEP instruction is really quite simple.
+`GetElementPtr <LangRef.html#getelementptr-instruction>`_ (GEP) instruction.
+Questions about the wily GEP instruction are probably the most frequently
+occurring questions once a developer gets down to coding with LLVM. Here we lay
+out the sources of confusion and show that the GEP instruction is really quite
+simple.
 
 Address Computation
 ===================
 
 When people are first confronted with the GEP instruction, they tend to relate
 it to known concepts from other programming paradigms, most notably C array
 indexing and field selection. GEP closely resembles C array indexing and field
 selection, however it is a little different and this leads to the following

 Can I do type-based alias analysis on LLVM IR?
 ----------------------------------------------
 
 You can't do type-based alias analysis using LLVM's built-in type system,
 because LLVM has no restrictions on mixing types in addressing, loads or stores.
 
 LLVM's type-based alias analysis pass uses metadata to describe a different type
 system (such as the C type system), and performs type-based aliasing on top of
-that.  Further details are in the `language reference <LangRef.html#tbaa>`_.
+that.  Further details are in the
+`language reference <LangRef.html#tbaa-metadata>`_.
 
 What happens if a GEP computation overflows?
 --------------------------------------------
 
 If the GEP lacks the ``inbounds`` keyword, the value is the result from
 evaluating the implied two's complement integer computation. However, since
 there's no guarantee of where an object will be allocated in the address space,
 such values have limited meaning.