[dev.simd] simdgen: filter arrangement symbols for shaped ARM64 instructions

For shaped instructions (Long, Narrow, Wide), extract arrangement
strings only from the relevant (input) explanation symbol. This avoids
output-side arrangements that instantiate() would spuriously widen/narrow.

For example, UMULL's XML includes <Ta>=2D (output) alongside the
input arrangements; collecting 2D caused spurious elemBits=128 defs.

No generated output changes — the spurious defs were already masked by
YAML. This fix is needed for follow-up CLs with PMULL and UADDW support.

Change-Id: I7583307bb5dc5ef543424c5adafc2b5350cab6ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/783900
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: David Chase <drchase@google.com>