[llvm-mca][docs] Add instruction flow documentation. NFC.
[llvm-mca][docs] Add instruction flow documentation. NFC.

Summary:
This patch mostly copies the existing Instruction Flow, and stage descriptions
from the mca README.  I made a few text tweaks, but no semantic changes,
and made reference to the "default pipeline."  I also removed the internals
references (e.g., reference to class names and header files).  I did leave the
LSUnit name around, but only as an abbreviated word for the load-store unit.


Reviewers: andreadb, courbet, RKSimon, gbedwell, filcab

Reviewed By: andreadb

Subscribers: tschuett, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D49692

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@338319 91177308-0d34-0410-b5e6-96231b3b80d8

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