[NFC][BdVer2] Tests for load and store throughput (PR39465)

During review it was noted that while it appears that
the Piledriver can do two [consecutive] loads per cycle,
it can only do one store per cycle. It was suggested
that the sched model incorrectly models that,
but it was opted to fix this afterwards.

These tests show that the two consecutive loads are
modelled correctly, and one consecutive stores is not
modelled incorrectly. Unless i'm missing the point.

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

llvm-svn: 346404