Remove lowerAffineConstructs and lowerControlFlow in favor of providing patterns.

These methods don't compose well with the rest of conversion framework, and create artificial breaks in conversion. Replace these methods with two(populateAffineToStdConversionPatterns and populateLoopToStdConversionPatterns respectively) that populate a list of patterns to perform the same behavior.

PiperOrigin-RevId: 258219277