LSU, Loadunit: Set all data to 0 when exception (#2888)

When an exception occurs, as s3_out is true, backend will still consider
the data valid at this time (which is actually in X state) valid and
bypass it. Although in real chips, this situation will be handled after
2 cycles of exception processing (flushing the pipeline and
redirecting), when using vcs simulation, the simulation cannot continue
due to the problem of X-state propagation. Therefore, when an exception
occurs in load_s3, the output data will be forcibly set to 0.

src/main/scala/xiangshan/mem/pipeline/LoadUnit.scala

@@ -1346,8 +1346,9 @@ class LoadUnit(implicit p: Parameters) extends XSModule
 
   // FIXME: add 1 cycle delay ?
   // io.lsq.uncache.ready := !s3_valid
+  val s3_outexception = ExceptionNO.selectByFu(s3_out.bits.uop.exceptionVec, LduCfg).asUInt.orR && s3_vecActive
   io.ldout.bits        := s3_ld_wb_meta
-  io.ldout.bits.data   := Mux(s3_valid, s3_ld_data_frm_cache, s3_ld_data_frm_uncache)
+  io.ldout.bits.data   := Mux(s3_valid, Mux(!s3_outexception, s3_ld_data_frm_cache, 0.U), s3_ld_data_frm_uncache)
   io.ldout.valid       := (s3_out.valid || (s3_mmio.valid && !s3_valid)) && !s3_vecout.isvec
 
   // TODO: check this --hx