arch-arm: Fix reg dependency for SVE gather microops

arch-arm: Fix reg dependency for SVE gather microops

The first microop of an SVE gather creates a copy of the
source vecreg into AA64FpUreg0. The subsequent microops
must refer to this copy as a source in order to establish
the correct register dependencies.

Change-Id: I84d8c331f9f9ebca609948a15f686a7cde67dc31
Signed-off-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19172
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>