[mlir][Vector] Introduce 'vector.load' and 'vector.store' ops

This patch adds the 'vector.load' and 'vector.store' ops to the Vector
dialect [1]. These operations model *contiguous* vector loads and stores
from/to memory. Their semantics are similar to the 'affine.vector_load' and
'affine.vector_store' counterparts but without the affine constraints. The
most relevant feature is that these new vector operations may perform a vector
load/store on memrefs with a non-vector element type, unlike 'std.load' and
'std.store' ops. This opens the representation to model more generic vector
load/store scenarios: unaligned vector loads/stores, perform scalar and vector
memory access on the same memref, decouple memory allocation constraints from
memory accesses, etc [1]. These operations will also facilitate the progressive
lowering of both Affine vector loads/stores and Vector transfer reads/writes
for those that read/write contiguous slices from/to memory.

In particular, this patch adds the 'vector.load' and 'vector.store' ops to the
Vector dialect, implements their lowering to the LLVM dialect, and changes the
lowering of 'affine.vector_load' and 'affine.vector_store' ops to the new vector
ops. The lowering of Vector transfer reads/writes will be implemented in the
future, probably as an independent pass. The API of 'vector.maskedload' and
'vector.maskedstore' has also been changed slightly to align it with the
transfer read/write ops and the vector new ops. This will improve reusability
among all these operations. For example, the lowering of 'vector.load',
'vector.store', 'vector.maskedload' and 'vector.maskedstore' to the LLVM dialect
is implemented with a single template conversion pattern.

[1] https://llvm.discourse.group/t/memref-type-and-data-layout/

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D96185

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