arch-arm: Fix SVE indx inst by sizeof error and dest overwrite

arch-arm: Fix SVE indx inst by sizeof error and dest overwrite

This patch includes two fixes for SVE FMUL; FMLA FMLS AND FCMLA instructions

+ Fixes indexed functions like FMUL, FMLA, FMLS, FCMLA due to its
destination register overwrite with temporary values, wince the imm
can make changes in vector positions that will be read in the future.

+ sizeof return bytes not bits so division of 128 shouild be of 16 instead

Change-Id: I304d1b254a299069c85bbc3319e5a6d4119436d0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28228
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>