[MC] Move the instruction latency computation from TargetSchedModel to MCSchedModel.

The goal is to make the latency information accessible through the MCSchedModel
interface. This is particularly important for tools like llvm-mca that only have
access to the MCSchedModel API.

This partially fixes PR36676.
No functional change intended.

Differential Revision: https://reviews.llvm.org/D44383

llvm-svn: 327406