[docs] Fix TableGen/LangRef typos

This fixes:
 - `Pred` -> `Prep`
 - `IfDef` -> `Ifdef` (rst is case sensitive here)

llvm-svn: 363294
diff --git a/llvm/docs/TableGen/LangRef.rst b/llvm/docs/TableGen/LangRef.rst
index 195106a..5079bc6 100644
--- a/llvm/docs/TableGen/LangRef.rst
+++ b/llvm/docs/TableGen/LangRef.rst
@@ -482,9 +482,9 @@
            : "#else" (`WhiteSpaceOrAnyComment`)* `LineEnd`
    PrepEndif: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
             : "#endif" (`WhiteSpaceOrAnyComment`)* `LineEnd`
-   PrepRegContentException: `PredIfdef` | `PredElse` | `PredEndif` | EOF
+   PrepRegContentException: `PrepIfdef` | `PrepElse` | `PrepEndif` | EOF
    PrepRegion: .* - `PrepRegContentException`
-             :| `PrepIfDef`
+             :| `PrepIfdef`
              :  (`PrepRegion`)*
              :  [`PrepElse`]
              :  (`PrepRegion`)*