[llvm-exegesis] Refactor InstructionBenchmark to Benchmark

When llvm-exegesis was first introduced, it only supported benchmarking
individual instructions, hence the name for the data structure storing
the data corresponding to a benchmark being called InstructionBenchmark
made sense. However, now that benchmarking arbitrary snippets is
supported, InstructionBenchmark doesn't correspond to a single
instruction. This patch refactors InstructionBenchmark to be called
Benchmark to clean up this little bit of technical debt.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D146884