target/riscv: Prevent lost illegal instruction exceptions

target/riscv: Prevent lost illegal instruction exceptions

When decode_insn16() fails, we fall back to decode_RV32_64C() for
further compressed instruction decoding. However, prior to this change,
we did not raise an illegal instruction exception, if decode_RV32_64C()
fails to decode the instruction. This means that we skipped illegal
compressed instructions instead of raising an illegal instruction
exception.

Instead of patching decode_RV32_64C(), we can just remove it,
as it is dead code since f330433b363 anyway.

Signed-off-by: Georg Kotheimer <georg.kotheimer@kernkonzept.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210322121609.3097928-1-georg.kotheimer@kernkonzept.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>