fastmodel: Ensure unset vec reg bits are zero/false.
fastmodel: Ensure unset vec reg bits are zero/false.

These bits won't be overwritten with values from IRIS, and so we should
make sure they're cleared and don't have old values or junk.

Change-Id: Ib81780ab523f00d6a4d31841d68a3d83924982a9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24327
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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