[flang][hlfir] Enable WHERE scheduling in LowerHLFIROrderedAssignments

Nothing special is needed, other than adding the logging code for where
masks and to plug the pattern. This tests mainly adds test.

Note that some of the justifications to create temps shows some lacks of
side effect interface on operations (like hlfir.transpose), or on some
transparent llvm intrinsic calls (llvm.stacksave/restore).
I think we should as much as possible try to improve this on the ops
generate code rather than special casing it here.

Differential Revision: https://reviews.llvm.org/D150581