MIR Serialization: Serialize the unnamed basic block references.

MIR Serialization: Serialize the unnamed basic block references.

This commit serializes the references from the machine basic blocks to the
unnamed basic blocks.

This commit adds a new attribute to the machine basic block's YAML mapping
called 'ir-block'. This attribute contains the actual reference to the
basic block.

Reviewers: Duncan P. N. Exon Smith

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243340 91177308-0d34-0410-b5e6-96231b3b80d8