[InstCombine] PHI-aware aggregate reconstruction: correctly detect "use" basic block

While the original implementation added in D85787 / ae7f0881
is not incorrect, it is known to be suboptimal.

In particular, it is not incorrect to use the basic block
in which the original `insertvalue` instruction is located
as the merge point, that is not necessarily optimal,
as `@test6` shows.

We should look at all the AggElts, and, if they are all defined
in the same basic block, then that is the basic block we should use.

On RawSpeed library, this catches +4% (+50) more cases.
On vanilla LLVM test-suits, this catches +12% (+92) more cases.