[ForwardOpTree] Reload know values.

For scalar accesses, change the access target to an array element that
is known to contain the same value.

This may become an alternative to forwardKnownLoad which creates new
loads (and therefore closer to forwarding speculatives). Reloading does
not require the known value originating from a load, but can be a store
as well.

Differential Revision: https://reviews.llvm.org/D39325

llvm-svn: 316766