arch-arm: Fix usage of RegId constructor for VecElem
arch-arm: Fix usage of RegId constructor for VecElem

RegId() constructor requires three arguments in case of a VecElem
register: the class, the vector index and the element index inside the
vector, otherwise it panics.

Change-Id: Ic842df4dcddaffa83b211aa6e1dd1953cafa4951
Signed-off-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-by: Giacomo Gabrielli <994b1a8ccf1e7b92fe64d091082ee5c7e191855a@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15615
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>

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