[DAGISel][ARM] Fix vector truncate combine for big-endian (#118101)

This DAG combine was incorrect for big-endian targets, because it
assumes that when a bitcast changes the lane width, the
least-

[DAGISel][ARM] Fix vector truncate combine for big-endian (#118101)

This DAG combine was incorrect for big-endian targets, because it
assumes that when a bitcast changes the lane width, the
least-significant bits of the wider lanes are in the lower-numbered
lanes of the smaller type, which is only true for little-endian.

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