This patch extends ArmSMEToSCF to support lowering of masked tile_load
ops. Only masks created by 'vector.create_mask' are currently supported.
There are two lowerings depending on the pad.
For pad of constant zero, the tile is first zeroed, then only active
rows are loaded.
For non-zero pad, the scalar pad is broadcast to a 1-D vector and a
regular 'vector.masked_load' (will be lowered to SVE, not SME) loads
each slice, with padding specified as a passthru and the 2-D mask
combined into a 1-D mask. The resulting slice is then inserted into the
tile with 'arm_sme.move_vector_to_tile_slice'.
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