clean up shift instruction implementation

riscv/insns/slli.h

@@ -1,3 +1,2 @@
-if (SHAMT >= xlen)
-  throw trap_illegal_instruction();
+require(SHAMT < xlen);
 WRITE_RD(sext_xlen(RS1 << SHAMT));

riscv/insns/srai.h

@@ -1,8 +1,2 @@
-if (xlen == 64)
-  WRITE_RD(sreg_t(RS1) >> SHAMT);
-else
-{
-  if(SHAMT & 0x20)
-    throw trap_illegal_instruction();
-  WRITE_RD(sext32(int32_t(RS1) >> SHAMT));
-}
+require(SHAMT < xlen);
+WRITE_RD(sext_xlen(sext_xlen(RS1) >> SHAMT));

riscv/insns/srl.h

@@ -1,4 +1 @@
-if (xlen == 64)
-  WRITE_RD(RS1 >> (RS2 & 0x3F));
-else
-  WRITE_RD(sext32((uint32_t)RS1 >> (RS2 & 0x1F)));
+WRITE_RD(sext_xlen(zext_xlen(RS1) >> (RS2 & (xlen-1))));

riscv/insns/srli.h

@@ -1,8 +1,2 @@
-if (xlen == 64)
-  WRITE_RD(RS1 >> SHAMT);
-else
-{
-  if(SHAMT & 0x20)
-    throw trap_illegal_instruction();
-  WRITE_RD(sext32((uint32_t)RS1 >> SHAMT));
-}
+require(SHAMT < xlen);
+WRITE_RD(sext_xlen(zext_xlen(RS1) >> SHAMT));