Simplify MIR Output used for Codegen Testing

- MIRYamlMapping: Default value provided for fields which have optional
mappings. Implemented == operators for required classes. When a field's value is
same as default value specified YAML IO class will not print it.

- MIRPrinter: Above mentioned behaviour is not on by default. If -simplify-mir
option not specified, then make yaml::Output to print fields with default values
too.

Differential Revision: https://reviews.llvm.org/D32304

llvm-svn: 302984