Split predictable mem opts into two different passes, one that runs b…

[gottesmm]

…efore diagnostics and one that runs after diagnostics.

I discovered while updating PMO for ownership that for ~5 years there has been a
bug where we were treating copy_addr of trivial values like an "Assign" (in PMO
terminology) of a non-trivial value and thus stopping allocation
elimination. When I fixed this I discovered that this caused us to no longer
emit diagnostics in a predictable way. Specifically, consider the following
swift snippet:

var _: UInt = (-1) >> 0

Today, we emit a diagnostic that -1 can not be put into a UInt. This occurs
since even though the underlying allocation is only stored into, the copy_addr
assign keeps it alive, causing the diagnostics pass to see the conversion. With
my fix though, we see that we are only storing into the allocation, causing the
allocation to be eliminated before the constant propagation diagnostic pass
runs, causing the diagnostic to no longer be emitted.

We should truly not be performing this type of DCE before we emit such
diagnostics. So in this commit, I split the pass into two parts:

1. A load promotion pass that performs the SSA formation needed for SSA based
   diagnostics to actually work.

2. An allocation elimination passes that run /after/ SSA based diagnostics.

This should be NFC since the constant propagation SSA based diagnostics do not
create memory operations so the output should be the same.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[gottesmm]

@swift-ci smoke test

[atrick]

Running PMO again seems fine since you already eliminated PMO from the optimization pipeline. As long as you need to run it twice for a different purpose each time, it's good that they have different pass names now.