[PowerPC/MIR Serialization] Target flags serialization support

[PowerPC/MIR Serialization] Target flags serialization support

Add support for MIR serialization of PowerPC-specific operand target flags
(based on the generic infrastructure added in r244185 and r245383).

I won't even pretend that this is good test coverage, but this includes the
regression test associated with r246372. Adding an MIR test for that fix is far
superior to adding an IR-level test because particular instruction-scheduling
decisions are necessary in order to expose the bug, and using an MIR test we
can start the pipeline post-scheduling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246373 91177308-0d34-0410-b5e6-96231b3b80d8