[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