[mlir] Dialect Conversion: Add support for post-order legalization order (#166292)

By default, the dialect conversion driver processes operations in
pre-order: the initial worklist is populated pre-order. (New/modified
operations are immediately legalized recursively.)

This commit adds a new API for selective post-order legalization.
Patterns can request an operation / region legalization via
`ConversionPatternRewriter::legalize`. They can call these helper
functions on nested regions before rewriting the operation itself.

Note: In rollback mode, a failed recursive legalization typically leads
to a conversion failure. Since recursive legalization is performed by
separate pattern applications, there is no way for the original pattern
to recover from such a failure.
GitOrigin-RevId: a38e0942407ef3395264831ef971838b69d2a652