Remove -simplify-libcalls pass form Passes documentation

This pass was removed in r184459.

Also added note that the InstCombine pass does library call
simplification.

Patch slightly modified from one by Daniel Liew
<[email protected]>!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206650 91177308-0d34-0410-b5e6-96231b3b80d8

Author: rnk

docs/Passes.rst

@@ -540,6 +540,8 @@ instructions that are obviously dead.
 A trivial dead store elimination that only considers basic-block local
 redundant stores.
 
+.. _passes-functionattrs:
+
 ``-functionattrs``: Deduce function attributes
 ----------------------------------------------
 
@@ -648,7 +650,7 @@ program, and is used for a wide variety of program transformations.
 ------------------------------------------------
 
 Combine instructions to form fewer, simple instructions.  This pass does not
-modify the CFG This pass is where algebraic simplification happens.
+modify the CFG. This pass is where algebraic simplification happens.
 
 This pass combines things like:
 
@@ -681,6 +683,13 @@ program:
    shifts.
 #. … etc.
 
+This pass can also simplify calls to specific well-known function calls (e.g.
+runtime library functions).  For example, a call ``exit(3)`` that occurs within
+the ``main()`` function can be transformed into simply ``return 3``. Whether or
+not library calls are simplified is controlled by the
+:ref:`-functionattrs <passes-functionattrs>` pass and LLVM's knowledge of
+library calls on different targets.
+
 ``-internalize``: Internalize Global Symbols
 --------------------------------------------
 
@@ -1011,14 +1020,6 @@ as:
 Note that this pass has a habit of making definitions be dead.  It is a good
 idea to run a :ref:`DCE <passes-dce>` pass sometime after running this pass.
 
-``-simplify-libcalls``: Simplify well-known library calls
----------------------------------------------------------
-
-Applies a variety of small optimizations for calls to specific well-known
-function calls (e.g. runtime library functions).  For example, a call
-``exit(3)`` that occurs within the ``main()`` function can be transformed into
-simply ``return 3``.
-
 .. _passes-simplifycfg:
 
 ``-simplifycfg``: Simplify the CFG