[mlir][LLVM] Add `SplitGEP` type-consistency pattern

The goal of this pattern is to eliminate all GEPs that have more than two indices by splitting it into multiple GEPs.
The advantage of this change is that the resulting GEPs only ever index into one aggregate at the time. This enables handling sub-aggregates in other patterns and also creates IR with easier to deduce pointer element types.

As a minor note, `getResultPtrElementType` for `GEPOp` was rewritten since it did not properly handle dynamic indices. The way GEPOp is specified, the resulting pointer element type can *always* be deduced from its base type and indices.

Differential Revision: https://reviews.llvm.org/D154692