Revert "[xcc, sim] added slei/sleui in lieu of slti/sltiu"

This reverts commit bf5406d4df625678bc6ec20ce1d48541541dba54.

We found a clever way to efficiently implement slti/sltiu despite the
reversed operands.  The trick is because of the following fact:

(a < b) === !(b <= a) === !(b-1 < a)

So just turn off the carry-in when doing the subtraction for the comparison.

riscv/execute.h

@@ -480,12 +480,12 @@ switch((insn.bits >> 0x19) & 0x7f)
       }
       case 0x2:
       {
-        #include "insns/slei.h"
+        #include "insns/slti.h"
         break;
       }
       case 0x3:
       {
-        #include "insns/sleiu.h"
+        #include "insns/sltiu.h"
         break;
       }
       case 0x4:

riscv/insns/slei.h

@@ -1 +0,0 @@
-RA = !(sreg_t(cmp_trunc(SIMM)) < sreg_t(cmp_trunc(RB)));

riscv/insns/sleiu.h

@@ -1 +0,0 @@
-RA = !(cmp_trunc(SIMM) < cmp_trunc(RB));

riscv/insns/slti.h

@@ -0,0 +1 @@
+RA = sreg_t(cmp_trunc(RB)) < sreg_t(cmp_trunc(SIMM));

riscv/insns/sltiu.h

@@ -0,0 +1 @@
+RA = cmp_trunc(RB) < cmp_trunc(SIMM);