[SelectionDAGBuilder] Chain prefetches less aggressively.

Prefetches used to always be chained between any previous and following
memory accesses. The problem with this was that later optimizations, such as
folding of a load into the user instruction, got disrupted.

This patch relaxes the chaining of prefetches in order to remedy this.

Reveiw: Hal Finkel
https://reviews.llvm.org/D38886

llvm-svn: 322163