EDSC: use affine.load/store instead of std.load/store

Standard load and store operations are evolving to be separated from the Affine
constructs.  Special affine.load/store have been introduced to uphold the
restrictions of the Affine control flow constructs on their operands.
EDSC-produced loads and stores were originally intended to uphold those
restrictions as well so they should use affine.load/store instead of
std.load/store.

PiperOrigin-RevId: 257443307