[mlir][Transforms][NFC] Simplify function signatures (#155997)

Many internal functions take a `ConversionPatternRewriter &` or
`ConversionPatternRewriterImpl &` as a parameter. There's only a single
instance of these classes, so it's better to store the reference in a
field. This commit is in preparation of another PR that will require
access to `ConversionPatternRewriter` in additional helper functions.

Note: Public API does not change.

Author: matthias-springer