[mlir][Vector] Fix vector.transfer alignment calculation

https://reviews.llvm.org/D79246 introduces alignment propagation for vector transfer operations. Unfortunately, the alignment calculation is incorrect and can result in crashes.

This revision fixes the calculation by using the natural alignment of the memref elemental type, instead of the resulting vector type.

If more alignment is desired, it can be done in 2 ways:
1. use a proper vector.type_cast to transform a memref<axbxcxdxf32> into a memref<axbxvector<cxdxf32>> giving a natural alignment of vector<cxdxf32>
2. add an alignment attribute to vector transfer operations and propagate it.

With this change the alignment in the relevant tests goes down from 128 to 4.

Lastly, a few minor cleanups are performed and the custom `isMinorIdentityMap` is deprecated.

Differential Revision: https://reviews.llvm.org/D80734