cpu: O3 rename using the flatIndex instead of index
cpu: O3 rename using the flatIndex instead of index

This patch is replacing the RegId::index with RegId::flatIndex so that
it provides a valid register number when used by a VecElem register.

Change-Id: I5b000abb9457cd325c2a3021e772a75ea33d8a4c
Signed-off-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15600
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>

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