Optimise SAR to not use from/toTwos conversion

Author: axic

lib/opFns.js

@@ -179,15 +179,16 @@ module.exports = {
     if (!runState._common.gteHardfork('constantinople')) {
       trap(ERROR.INVALID_OPCODE)
     }
+    const isSigned = b.testn(255)
     if (a.gten(256)) {
-      if (b.fromTwos(256).ltn(0)) {
-        return new BN(-1).toTwos(256)
+      if (isSigned) {
+        return new BN(utils.MAX_INTEGER)
       } else {
         return new BN(0)
       }
     }
     const c = b.shrn(a.toNumber())
-    if (b.testn(255)) {
+    if (isSigned) {
       const shiftedOutWidth = 255 - a.toNumber()
       const mask = utils.MAX_INTEGER.shrn(shiftedOutWidth).shln(shiftedOutWidth)
       return c.ior(mask)