[llvm-exegesis][NFC] Return many CodeTemplates instead of one.

Summary: This is part one of the change where I simply changed the signature of the functions. More work need to be done to actually produce more than one CodeTemplate per instruction.

Reviewers: courbet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D53209

llvm-svn: 344493