[AArch64][GISel] Don't pointlessly lower G_TRUNC (#81479)

If we have something like G_TRUNC from v2s32 to v2s16, then lowering
this to a concat of two G_TRUNC s32 to s16 followed by G_TRUNC from
v2s16 to v2s8 does not bring us any closer to legality. In fact, the
first part of that is a G_BUILD_VECTOR whose legalization will produce a
new G_TRUNC from v2s32 to v2s16, and both G_TRUNCs will then get
combined to the original, causing a legalization cycle.

Make the lowering condition more precise, by requiring that the original
vector is >128 bits, which is I believe the only case where this
specific splitting approach is useful.

Note that this doesn't actually produce a legal result (the alwaysLegal
is a lie, as before), but it will cause a proper globalisel abort
instead of an infinite legalization loop.

Fixes https://github.com/llvm/llvm-project/issues/81244.