[Improve CodeGen Testing] This patch renables MIRPrinter print fields which have value equal to its default.
If -simplify-mir option is passed then MIRPrinter will not print such fields.
This change also required some lit test cases in CodeGen directory to be changed.

Reviewed By: MatzeB

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

llvm-svn: 304779