Revamp StableHLO simplification paterns.

[GleasonK]

Starting to look into porting new simplification patterns from MHLO to StableHLO and wanted to clean things up in the process to make StableHLO simplification/folders an overall better place to innovate:

• Leverage DRR when possible. This is usually possible when the pattern isn't super complicated or the op being built doesn't have a region.
• Order simplification patterns by op alphabetically, order tests alphabetically, ensure a test per simplification pattern.
• Add code comments for each pattern of the format // Pattern: ... which can be used in the future for generating documentation.
• Move patterns which fold constants to folder pass.