riscv: throw IllegalInstFault when decoding invalid instructions

riscv: throw IllegalInstFault when decoding invalid instructions

If an instruction is invalid, some assertions may in the decoder may
fail the entire simulation. Instead, we want to raise an
IllegalInstFault instead of failing immediately in the decoder if the
invalid instruction is being speculatively executed.

Change-Id: I5cb72ba06f07f173922f86897ddfdf677e8c702f
Reviewed-on: https://gem5-review.googlesource.com/9261
Maintainer: Alec Roelke <ar4jc@virginia.edu>
Reviewed-by: Monir Zaman <monir.zaman.m@gmail.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>