[X86][Basically NFC] Sched: split WriteBitScan into WriteBSF/WriteBSR.

Summary:
Motivation: {F6597954}

This only does the mechanical splitting, does not actually change
any numbers, as the tests added in previous revision show.

Reviewers: craig.topper, RKSimon, courbet

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48998

llvm-svn: 336511