[mlir] Add canonicalization for cond_br that feed into a cond_br on the same condition

[rriddle]

Summary

  ...
  cond_br %cond, ^bb1(...), ^bb2(...)
...
^bb1: // has single predecessor
  ...
  cond_br %cond, ^bb3(...), ^bb4(...)

->

  ...
  cond_br %cond, ^bb1(...), ^bb2(...)
...
^bb1: // has single predecessor
  ...
  br ^bb3(...)

[jpienaar]

Nice, micro nit: in description add ... as when I initially read it I thought there was requirement for empty block bb1

[mehdi_amini]

mlir/lib/Dialect/StandardOps/IR/Ops.cpp

1104

Trivial braces?

[bondhugula]

mlir/lib/Dialect/StandardOps/IR/Ops.cpp

1067

Consider adding a line on the simplification - the example below doesn't capture the fact that ^bb1 has only one predessor.

1076

Nit: please add `// has single predecessor```

1097

Nit: a -> an

[rriddle]

Resolve comments

[rriddle]

Thanks for the review!