[AArch64] Improve lowering of truncating uzp1 (#82457)

There were two existing patterns:
    `concat_vectors(trunc(x), trunc(y)) -> uzp1(x, y)`
`concat_vectors(assertzext(trunc(x)), assertzext(trunc(y))) -> uzp1(x,
y)`

Move them into a class and add the following `assertsext` pattern to it:
`concat_vectors(assertsext(trunc(x)), assertsext(trunc(y))) -> uzp1(x,
y)`

Add the following transform for v8i8 and v4i16 result types to help with
pattern matching:
  `truncating uzp1(x, y) -> trunc(concat(x, y))`
And a pattern to go with it:
  `trunc(concat_vectors(x, y)) -> uzp1 (x, y)`

Add another isel pattern for v8i8 and v4i16 result vector types, similar
to
the existing concat pattern, but with a trunc node in the begining:
`trunc(concat_vectors(assertext_trunc(x), assertext_trunc(y))) ->
xtn(uzp1(x, y))`