Fix register trashing in Emit*ByteSeqStringSetChar

test/mjsunit/regress/regress-crbug-320922.js

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 // Flags: --allow-natives-syntax
 
-var do_set = false;
+var string = "hello world";
+var expected = "Hello " + "world";
+function Capitalize() {
+  %_OneByteSeqStringSetChar(string, 0, 0x48);
+}
+Capitalize();
+assertEquals(expected, string);
+Capitalize();
+assertEquals(expected, string);
 
-%NeverOptimizeFunction(set_proto_elements);
-function set_proto_elements() {
-  if (do_set) Array.prototype[1] = 1.5;
+var twobyte = "\u20ACello world";
+
+function TwoByteCapitalize() {
+  %_TwoByteSeqStringSetChar(twobyte, 0, 0x48);
 }
-
-function f(a, i) {
-  set_proto_elements();
-  return a[i] + 0.5;
-}
-
-var arr = [0.0,,2.5];
-assertEquals(0.5, f(arr, 0));
-assertEquals(0.5, f(arr, 0));
-%OptimizeFunctionOnNextCall(f);
-assertEquals(0.5, f(arr, 0));
-do_set = true;
-assertEquals(2, f(arr, 1));
-
+TwoByteCapitalize();
+assertEquals(expected, twobyte);
+TwoByteCapitalize();
+assertEquals(expected, twobyte);