[pmo] Eliminate incomplete support for promoting enums.

[gottesmm]

This was never implemented correctly way back in 2013-2014. It was originally
added I believe so we could DI checks, but the promotion part was never added.

Given that DI is now completely split from PMO, we can just turn this off and if
necessary add it back on master "properly".

rdar://41161408

[gottesmm]

@swift-ci smoke test and merge

[gottesmm]

I am fine with either reviewers.

[atrick]

@gottesmm your change looks safe. I'm not quite sure where it was broken and will have to trust you that it was never kicking in downstream somewhere. If you wanted to be sure you could just compare the stripped SIL from the benchmarks.

[gottesmm]

@atrick let me elaborate a bit. The problem is that code for handling this wasn't added in a bunch of places in Predictable Mem Opts. A few examples:

• We do not have a way to distinguish in between different cases of enums. You can see this by how we represent the flattened type tree here:
  https://github.com/apple/swift/blob/master/lib/SILOptimizer/Mandatory/PredictableMemOpt.cpp#L46
• We do not know how to promote the init_enum_data_addr to an unchecked_enum_data here:
  https://github.com/apple/swift/blob/master/lib/SILOptimizer/Mandatory/PredictableMemOpt.cpp#L294
• We do not know how to look through said init_enum_data_addr here:
  https://github.com/apple/swift/blob/master/lib/SILOptimizer/Mandatory/PredictableMemOpt.cpp#L101
  That was the assert that we were hitting.

[atrick]

@gottesmm Yeah, the code you removed is pretty obviously not supposed to run in this pass!