[llvm] [InstCombine] Extend folding of aggregate construction to cases when source aggregates are partially available (PR #100828)

[via llvm-commits]

weiguozhi wrote:

Add new instcombine pattern to transform
```
         extract (select (cond, insert(agg, elem), FV))
     ->  select (cond, elem, extract(FV))
```
so the regression of https://github.com/dtcxzyw/llvm-opt-benchmark/pull/1351#discussion_r1802502189 can be fixed.

https://github.com/llvm/llvm-project/pull/100828