[mlir] SCF: provide function_ref builders for IfOp

Now that OpBuilder is available in `build` functions, it becomes possible to
populate the "then" and "else" regions directly when building the "if"
operation. This is desirable in more structured forms of builders, especially
in when conditionals are mixed with loops. Provide new `build` APIs taking
callbacks for body constructors, similarly to scf::ForOp, and replace more
clunky edsc::BlockBuilder uses with these. The original APIs remain available
and go through the new implementation.

Differential Revision: https://reviews.llvm.org/D80527