[llvm-exegesis] Add instructions to BenchmarkResult Key.

We want llvm-exegesis to explore instructions (effect of initial register values, effect of operand selection). To enable this a BenchmarkResult muststore all the relevant data in its key. This patch starts adding such data. Here we simply allow to store the generated instructions, following patches will add operands and initial values for registers.

https://reviews.llvm.org/D47764

Authored by: Guilluame Chatelet

llvm-svn: 334008