[X86][BdVer2] Fix loads/stores throughput for Piledriver (PR39465)

There are two AGU units, and per 1cy, there can be either two loads,
or a load and a store; but not two stores, or two loads and a store.

Additionally, loads shouldn't affect the store scheduler and vice versa.
(but *should* affect the PdEX scheduler.)

Required rL346545.
Fixes https://bugs.llvm.org/show_bug.cgi?id=39465

llvm-svn: 346587