Add RV32 restriction for compressed shift instructions

The restriction was present for `C.SLLI` but was missing for `C.SRLI` and `C.SRAI`.

The format is copied from `C.SLLI`.

Fixes #356

model/riscv_insts_cext.sail

@@ -224,9 +224,9 @@ mapping clause assembly = C_LUI(imm, rd)
 union clause ast = C_SRLI : (bits(6), cregidx)
 
 mapping clause encdec_compressed = C_SRLI(nzui5 @ nzui40, rsd)
-      if nzui5 @ nzui40 != 0b000000
+      if nzui5 @ nzui40 != 0b000000 & (sizeof(xlen) == 64 | nzui5 == 0b0)
   <-> 0b100 @ nzui5 : bits(1) @ 0b00 @ rsd : cregidx @ nzui40 : bits(5) @ 0b01
-      if nzui5 @ nzui40 != 0b000000
+      if nzui5 @ nzui40 != 0b000000 & (sizeof(xlen) == 64 | nzui5 == 0b0)
 
 function clause execute (C_SRLI(shamt, rsd)) = {
   let rsd = creg2reg_idx(rsd);
@@ -242,9 +242,9 @@ mapping clause assembly = C_SRLI(shamt, rsd)
 union clause ast = C_SRAI : (bits(6), cregidx)
 
 mapping clause encdec_compressed = C_SRAI(nzui5 @ nzui40, rsd)
-      if nzui5 @ nzui40 != 0b000000
+      if nzui5 @ nzui40 != 0b000000 & (sizeof(xlen) == 64 | nzui5 == 0b0)
   <-> 0b100 @ nzui5 : bits(1) @ 0b01 @ rsd : cregidx @ nzui40 : bits(5) @ 0b01
-      if nzui5 @ nzui40 != 0b000000
+      if nzui5 @ nzui40 != 0b000000 & (sizeof(xlen) == 64 | nzui5 == 0b0)
 
 function clause execute (C_SRAI(shamt, rsd)) = {
   let rsd = creg2reg_idx(rsd);