Sync-up SROA notes

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<p>SROA - We've re-written SROA to be significantly more powerful.

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<p>SROA - We&#8217;ve re-written SROA to be significantly more powerful and generate

code which is much more friendly to the rest of the optimization pipeline.

Previously this pass had scaling problems that required it to only operate on

relatively small aggregates, and at times it would mistakenly replace a large

aggregate with a single very large integer in order to make it a scalar SSA

value. The result was a large number of i1024 and i2048 values representing any

small stack buffer. These in turn slowed down many subsequent optimization

paths.</p>

<p>The new SROA pass uses a different algorithm that allows it to only promote to

scalars the pieces of the aggregate actively in use. Because of this it doesn’t

require any thresholds. It also always deduces the scalar values from the uses

of the aggregate rather than the specific LLVM type of the aggregate. These

features combine to both optimize more code with the pass but to improve the

compile time of many functions dramatically.</p>

<ul>

  <li>Branch weight metadata is preseved through more of the optimizer.</li>

  <li>...</li>

</ul>

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