[llvm-exegesis] Add predefined floating point values so we can test impact of...

[llvm-exegesis] Add predefined floating point values so we can test impact of special values on latency.

Summary: This will be useful to generate many configurations and test instruction regimes (NaN, Inf, subnormal, normal).

Reviewers: courbet

Subscribers: mgorny, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D51858

llvm-svn: 342369