[X86][SandyBridge] SBWriteResPair +5cy Memory Folds

As mentioned on D44647, this patch increases the default memory latency to +5cy , which more closely matches what most custom cases are doing for reg-mem instructions.

I've bumped LoadLatency, ReadAfterLd and WriteLoad values to 5cy to be consistent.

As Sandy Bridge is currently our default generic model, this affects a lot of scheduling tests...

Differential Revision: https://reviews.llvm.org/D44654

llvm-svn: 329388