[llvm] [llvm][docs] Add example use of `llvm-reduce`. (PR #73237)

[Matt Arsenault via llvm-commits]

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@@ -105,4 +105,33 @@ EXIT STATUS
 :program:`llvm-reduce` returns 0 under normal operation. It returns a non-zero
 exit code if there were any errors.
 
+EXAMPLE
+-------
 
+:program:`llvm-reduce` can be used to simplify a test that causes a
+compiler crash.
+
+For example, let's assume that `opt` is crashing on the IR file
+`test.ll` with error message `Assertion failed at line 1234 of
+WhateverFile.cpp`, when running at `-O2`.
+
+The test case of `test.ll` can be reduced by invoking the following
+command:
+
+.. code-block:: bash
+
+   $(LLVM_BUILD_FOLDER)/bin/llvm-reduce --test=script.sh <path to>/test.ll
+
+The shell script passed to the option `test` consists of the
+following:
+
+.. code-block:: bash
+
+   $(LLVM_BUILD_FOLDER)/bin/opt -O2 -S $1 -o /dev/null \
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arsenm wrote:

Would be good to use -disable-output instead of -o /dev/null 

https://github.com/llvm/llvm-project/pull/73237