Add support for instance specific pass statistics.

Statistics are a way to keep track of what the compiler is doing and how effective various optimizations are. It is useful to see what optimizations are contributing to making a particular program run faster. Pass-instance specific statistics take this even further as you can see the effect of placing a particular pass at specific places within the pass pipeline, e.g. they could help answer questions like "what happens if I run CSE again here".

Statistics can be added to a pass by simply adding members of type 'Pass::Statistics'. This class takes as a constructor arguments: the parent pass pointer, a name, and a description. Statistics can be dumped by the pass manager in a similar manner to how pass timing information is dumped, i.e. via PassManager::enableStatistics programmatically; or -pass-statistics and -pass-statistics-display via the command line pass manager options.

Below is an example:

struct MyPass : public OperationPass<MyPass> {
  Statistic testStat{this, "testStat", "A test statistic"};

  void runOnOperation() {
    ...
    ++testStat;
    ...
  }
};

$ mlir-opt -pass-pipeline='func(my-pass,my-pass)' foo.mlir -pass-statistics

Pipeline Display:
===-------------------------------------------------------------------------===
                         ... Pass statistics report ...
===-------------------------------------------------------------------------===
'func' Pipeline
  MyPass
    (S) 15 testStat - A test statistic
  MyPass
    (S)  6 testStat - A test statistic

List Display:
===-------------------------------------------------------------------------===
                         ... Pass statistics report ...
===-------------------------------------------------------------------------===
MyPass
  (S) 21 testStat - A test statistic

PiperOrigin-RevId: 284022014