MIR Serialization: Serialize the null register operands.

MIR Serialization: Serialize the null register operands.

This commit serializes the null register machine operands.
It uses the '_' keyword to represent them, but the parser
also allows the '%noreg' named register syntax.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10580

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240558 91177308-0d34-0410-b5e6-96231b3b80d8