[X86] Extract latency of fldz/fld1 in separate classes.

Summary:
 - I've measured the values for Broadwell, Haswell, SandyBridge, Skylake.
 - For ZnVer1 and Atom, values were transferred form `InstRW`s.
 - For SLM and BtVer2, values are from Agner.

This is split off from https://reviews.llvm.org/D47377

Reviewers: RKSimon, andreadb

Subscribers: gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D47523

llvm-svn: 333642