[Metarenamer] Use 'inst' as default name for instructions

Currently we use 'tmp', which is also a keyword for FileCheck. It leads to this
annoying warning whenever a script for auto-generation of checks is used.
It is especially annoying that it happens to every test affected by metarenamer.

Just use another prefix for metarenamed names to avoid this.

Differential Revision: https://reviews.llvm.org/D144001
Reviewed By: nikic

Author: xortator

llvm/lib/Transforms/Utils/MetaRenamer.cpp

@@ -115,7 +115,7 @@ void MetaRename(Function &F) {
 
     for (auto &I : BB)
       if (!I.getType()->isVoidTy())
-        I.setName("tmp");
+        I.setName("inst");
   }
 }