Auto merge of #110824 - cjgillot:const-prop-index, r=JakobDegen,oli-obk

ConstProp into PlaceElem::Index.

Noticed this while looking at keccak output MIR.

This pass aims to replace `ProjectionElem::Index` with `ProjectionElem::ConstantIndex` during ConstProp.

r? `@ghost`

Author: bors