[X86][Sched] A bunch of fixes to the Zen2 sched model latencies.

Summary:
As determined with `llvm-exegesis`.

Some of these look like typos/misunderstandings of the sched model td
spec:
  - latency defaults to `1` when not set => Maybe we can avoid
    having a default ?
  - problems with regexps not being anchored by default (XCHG matching
    CMPXHG)

Note that this is not complete, it fixes only the most obvious mistakes,
and only for latency (not uops).

Reviewers: RKSimon, GGanesh

Subscribers: hiraditya, jfb, mstojanovic, hfinkel, craig.topper, andreadb, lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73172