[PATCH] Allow tail call optimization through multiple/nested struct extractions/insertions

[Stephen Lin]

Hi,

This patch generalizes target-independent tail call opportunity
detection to handle cases where a struct is decomposed and recomposed
multiple times, is nested within another struct, or any combination of
both along with any combination of other intermediate no-ops. It also
allows tail calls when structs are partially undefined, as long as all
defined values have the appropriate origins.

Finally, it adds an explicit check for the all-zero getelementptr
case; it seems like the case already works most of the time due to
other reductions, but I'm adding an explicit check to minimize phase
ordering issues and because an in-development patch depends on it.

Please kindly review.
Stephen
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