[mlir][llvm] Return failure from type converter for n-D scalable vectors (#65450)

This patch changes vector type conversion to return failure on n-D
scalable vector types instead of asserting.

This is an alternative approach to #65261 that aims to enable lowering
of Vector ops directly to ArmSME intrinsics where possible, and seems
more consistent with other type conversions. It's trivial to hit the
assert at the moment and it could be interpreted as n-D scalable vector
types being a bug, when they're valid types in the Vector dialect.

By returning failure it will generally fail more gracefully,
particularly for release builds or other builds where assertions are
disabled.