Various improvements to pattern matching and other infra:

 - Make it so OpPointer implicitly converts to SSAValue* when the underlying op
   has a single value.  This eliminates a lot more ->getResult() calls and makes
   the behavior more LLVM-like
 - Fill out PatternBenefit to be typed instead of just a typedef for int with
   magic numbers.
 - Simplify various code due to these changes.

PiperOrigin-RevId: 217020717