MIR Serialization: Use the global value syntax for global value memory operands.

This commit modifies the serialization syntax so that the global IR values in
machine memory operands use the global value '@<name>' syntax instead of the
current '%ir.<name>' syntax.

The unnamed global IR values are handled by this commit as well, as the
existing global value parsing method can parse the unnamed globals already.

llvm-svn: 245527