arch-arm: just return the fault in twoEqualRegInst{,Fp}

arch-arm: just return the fault in twoEqualRegInst{,Fp}

This prevents the code from using the uninitialized destReg when
running the ``if (imm >= eCount)`` branch, which will make GCC 10.2
report a -Werror=maybe-uninitialized error when building gem5.opt.

Change-Id: Ie6e7d3d47a1b65b840b2106263ecfc21eb6af26b
Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34275
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>