[rtl] Fix handshake on compressed decoder

The handshake only considered whether the ID stage would be ready. But
the actual pipeline register will also take the `pc_set_i` signal into
account, which signals a jump. Since the compressed decoder has state
now (through the Zcmp extension), this improper handshake led to some of
the expanded instructions to get lost.

At the same time, we also take this signal into account for the enable
signal of the pipeline stage to avoid unnecessary switching.

Author: SamuelRiedel

rtl/ibex_if_stage.sv

@@ -415,7 +415,7 @@ module ibex_if_stage import ibex_pkg::*; #(
     .clk_i          (clk_i),
     .rst_ni         (rst_ni),
     .valid_i        (fetch_valid & ~fetch_err),
-    .id_in_ready_i  (id_in_ready_i),
+    .id_in_ready_i  (id_in_ready_i & ~pc_set_i),
     .instr_i        (if_instr_rdata),
     .instr_o        (instr_decompressed),
     .is_compressed_o(instr_is_compressed),
@@ -490,7 +490,7 @@ module ibex_if_stage import ibex_pkg::*; #(
   // Valid is held until it is explicitly cleared (due to an instruction completing or an exception)
   assign instr_valid_id_d = (if_instr_valid & id_in_ready_i & ~pc_set_i) |
                             (instr_valid_id_q & ~instr_valid_clear_i);
-  assign instr_new_id_d   = if_instr_valid & id_in_ready_i;
+  assign instr_new_id_d   = if_instr_valid & id_in_ready_i & ~pc_set_i;
 
   always_ff @(posedge clk_i or negedge rst_ni) begin
     if (!rst_ni) begin