[mlir][xegpu] Add OptimizeBlockLoads pass. (#165483)
This pass rewrites certain xegpu `CreateNd` and `LoadNd` operations that
feeds into `vector.transpose` to more optimal form to improve
performance. Specifically, low precision (bitwidth < 32) `LoadNd` ops
that feeds into transpose ops are rewritten to i32 loads with a valid
transpose layout such that later passes can use the load with transpose
HW feature to accelerate such load ops.
**Update:**
Pass is renamed to `OptimizeBlockLoads ` because later we plan to add
the array length optimization into this pass as well. This will break
down a larger load (like `32x32xf16`) into more DPAS-favorable array
length loads (`32x16xf16` with array length = 2). Both these
optmizations require rewriting `CreateNd` and `LoadNd` and it makes
sense to have a common pass for both.
