[MLIR] Implement emulation of static indexing subbyte type vector stores (#115922)

This patch enables unaligned, statically indexed storing of vectors with
sub emulation width element types.

To illustrate the mechanism, consider the example of storing
vector<7xi2> into memref<3x7xi2>[1, 0].
In this case the linearized indices of those bits being overwritten are
[14, 28), which are:

* the last 2 bits of byte no.2
* byte no.3
* first 4 bits of byte no.4

Because memory accesses are in bytes, byte no.2 and no.4 in the above
example are only being modified partially.
In the case of multi-threading scenario, in order to avoid data
contention, these two bytes must be handled atomically.

Author: lialan