[Docs] Switch a code block from LLVM to text

While I can't replicate this locally, it looks like the buildbots don't
recognize the IR block in r358385 l764 as IR. Downgrade it to being just
text while I look into it.

http://lab.llvm.org:8011/builders/llvm-sphinx-docs/builds/30132

llvm-svn: 358391

llvm/docs/SourceLevelDebugging.rst

@@ -761,7 +761,7 @@ straightforwards: determining the variable location in every basic block
 requries the consideraton of control flow. Consider the following IR, which
 presents several difficulties:
 
-.. code-block:: llvm
+.. code-block:: text
 
   define dso_local i32 @foo(i1 %cond, i32 %input) !dbg !12 {
   entry: