[mlir][FoldUtils] Ensure the created constant dominates the replaced op

This revision fixes a bug where an operation would get replaced with
a pre-existing constant that didn't dominate it. This can occur when
a pattern inserts operations to be folded at the beginning of the
constants insertion block. This revision fixes the bug by moving the
existing constant before the replaced operation in such cases. This is
fine because if a constant didn't already exist, a new one would have
been inserted before this operation anyways.

Differential Revision: https://reviews.llvm.org/D108498