[MLIR][LLVM][SROA] Make GEP handling type agnostic

This commit removes SROA's type consistency constraints from LLVM
dialect's GEPOp. The checks for valid indexing are now purely done by
computing the GEP's offset with the aid of the data layout.

To simplify handling of nested "subslots", we are tricking the SROA by
handing in memory slots that hold byte array types. This ensures that
subsequent accesses only need to check if their access will be
in-bounds. This lifts the requirement of detemining the sub-types for
all but the first level of subslots.