[MLIR] Add SCF.if Condition Canonicalizations

Add two canoncalizations for scf.if.
  1) A canonicalization that allows users of a condition within an if to assume the condition
     is true if in the true region, etc.
  2) A canonicalization that removes yielded statements that are equivalent to the condition
     or its negation

Differential Revision: https://reviews.llvm.org/D101012