[mlir] Move Conversion/StandardToStandard to Dialect/StandardOps/Transforms/FuncConversions

Conversion/ folders were originally intended to store patterns for
DialectA->DialectB conversions that depend on both dialects and do not
conceptually belong to either of the dialects. As such, DialectA->DialectA
conversion does not make sense under Conversion/ and should rather live with
the dialect it operates on.

Differential Revision: https://reviews.llvm.org/D79569