arch-arm: Fix decoding of LDFF1x scalar plus scalar
arch-arm: Fix decoding of LDFF1x scalar plus scalar

First-faulting loads do allow Rm == 0x1f.

Change-Id: Ib9bcb55e126653813fdbb7c29970af23a2471ebb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23803
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5