[instcombine] Do demanded elts last when visiting extractelement

This reorders existing transforms to put demanded elements last. The reasoning here is that when we have an example which can be scalarized or handled via demanded bits, we should prefer scalarization as that doesn't require dropping flags on arithmetic instructions.

This doesn't show major changes in the tests today, but once I add support for fast math flags to dropPoisonGeneratingFlags this becomes glaringly obvious.

Differential Revision: https://reviews.llvm.org/D115394