refactor(decoder): simplify interrupt indexing

The indexing expression was really complex.  It is constant so is is
possible to directly use an "unsized" localparam.  The values are less
than 32 so it is equivalent to previous code.  This coding style is the
one used in the CSR regfile.

It will simplify future modifications, including completing
parametrization.

core/decoder.sv

@@ -1344,53 +1344,29 @@ module decoder
       // we have three interrupt sources: external interrupts, software interrupts, timer interrupts (order of precedence)
       // for two privilege levels: Supervisor and Machine Mode
       // Supervisor Timer Interrupt
-      if (irq_ctrl_i.mie[riscv::S_TIMER_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]] && irq_ctrl_i.mip[riscv::S_TIMER_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]]) begin
+      if (irq_ctrl_i.mie[riscv::IRQ_S_TIMER] && irq_ctrl_i.mip[riscv::IRQ_S_TIMER]) begin
         interrupt_cause = riscv::S_TIMER_INTERRUPT;
       end
       // Supervisor Software Interrupt
-      if (irq_ctrl_i.mie[riscv::S_SW_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]] && irq_ctrl_i.mip[riscv::S_SW_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]]) begin
+      if (irq_ctrl_i.mie[riscv::IRQ_S_SOFT] && irq_ctrl_i.mip[riscv::IRQ_S_SOFT]) begin
         interrupt_cause = riscv::S_SW_INTERRUPT;
       end
       // Supervisor External Interrupt
       // The logical-OR of the software-writable bit and the signal from the external interrupt controller is
       // used to generate external interrupts to the supervisor
-      if (irq_ctrl_i.mie[riscv::S_EXT_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]] && (irq_ctrl_i.mip[riscv::S_EXT_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]] | irq_i[ariane_pkg::SupervisorIrq])) begin
+      if (irq_ctrl_i.mie[riscv::IRQ_S_EXT] && (irq_ctrl_i.mip[riscv::IRQ_S_EXT] | irq_i[ariane_pkg::SupervisorIrq])) begin
         interrupt_cause = riscv::S_EXT_INTERRUPT;
       end
       // Machine Timer Interrupt
-      if (irq_ctrl_i.mip[riscv::M_TIMER_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]] && irq_ctrl_i.mie[riscv::M_TIMER_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]]) begin
+      if (irq_ctrl_i.mip[riscv::IRQ_M_TIMER] && irq_ctrl_i.mie[riscv::IRQ_M_TIMER]) begin
         interrupt_cause = riscv::M_TIMER_INTERRUPT;
       end
       // Machine Mode Software Interrupt
-      if (irq_ctrl_i.mip[riscv::M_SW_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]] && irq_ctrl_i.mie[riscv::M_SW_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]]) begin
+      if (irq_ctrl_i.mip[riscv::IRQ_M_SOFT] && irq_ctrl_i.mie[riscv::IRQ_M_SOFT]) begin
         interrupt_cause = riscv::M_SW_INTERRUPT;
       end
       // Machine Mode External Interrupt
-      if (irq_ctrl_i.mip[riscv::M_EXT_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]] && irq_ctrl_i.mie[riscv::M_EXT_INTERRUPT[$clog2(
-              riscv::XLEN
-          )-1:0]]) begin
+      if (irq_ctrl_i.mip[riscv::IRQ_M_EXT] && irq_ctrl_i.mie[riscv::IRQ_M_EXT]) begin
         interrupt_cause = riscv::M_EXT_INTERRUPT;
       end