riscv: remove NonSpeculative flag from fence inst

Fence instruction origially had two flags NonSpeculative and
MemBarrier. In O3 model, MemBarrier instructions are inserted
into the instruction queue by the InstructionQueue::insertBarrier (at
src/cpu/o3/iew_impl.hh:1083). Barrier instructions are implicitly
assumed to be non-speculative.

Adding NonSpeculative flag to fence instruction makes it inserted into
the instruction queue twice (at src/cpu/o3/iew_impl.hh:1083 and :1111).
This can lead to a deadlock if both pointers to the instruction are not
cleared from the queue when the instruction retires.

This patch removes NonSpeculative flag from the fence inst.

Change-Id: I26573d12a0b52f43b73c0e51158286dc98d05ea4
Reviewed-on: https://gem5-review.googlesource.com/c/8183
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Alec Roelke <ar4jc@virginia.edu>
Maintainer: Alec Roelke <ar4jc@virginia.edu>

src/arch/riscv/isa/decoder.isa

@@ -411,7 +411,7 @@ decode QUADRANT default Unknown::unknown() {
         0x03: decode FUNCT3 {
             format IOp {
                 0x0: fence({{
-                }}, uint64_t, IsNonSpeculative, IsMemBarrier, No_OpClass);
+                }}, uint64_t, IsMemBarrier, No_OpClass);
                 0x1: fence_i({{
                 }}, uint64_t, IsNonSpeculative, IsSerializeAfter, No_OpClass);
             }