Issue #1894 - Part 1: Implement coalesce JS opcode

Based on https://bugzilla.mozilla.org/show_bug.cgi?id=1566141

js/src/jsopcodeinlines.h

@@ -22,6 +22,7 @@ GetDefCount(JSScript* script, unsigned offset)
      * in the pushed array of stack values for type inference.
      */
     switch (JSOp(*pc)) {
+      case JSOP_COALESCE:
       case JSOP_OR:
       case JSOP_AND:
         return 1;

js/src/vm/Interpreter.cpp

@@ -2128,6 +2128,16 @@ CASE(JSOP_IFNE)
 }
 END_CASE(JSOP_IFNE)
 
+CASE(JSOP_COALESCE)
+{
+    MutableHandleValue res = REGS.stackHandleAt(-1);
+    bool cond = !res.isNullOrUndefined();
+    if (cond) {
+        ADVANCE_AND_DISPATCH(GET_JUMP_OFFSET(REGS.pc));
+    }
+}
+END_CASE(JSOP_COALESCE)
+
 CASE(JSOP_OR)
 {
     bool cond = ToBoolean(REGS.stackHandleAt(-1));

js/src/vm/Opcodes.h

@@ -2315,7 +2315,7 @@
      *   Operands:
      *   Stack: =>
      */ \
-    macro(JSOP_JUMPTARGET,  230, "jumptarget",     NULL,  1,  0,  0,  JOF_BYTE)\
+    macro(JSOP_JUMPTARGET,  230, "jumptarget",     NULL,  1,  0,  0,  JOF_BYTE) \
     /*
      * Like JSOP_CALL, but tells the function that the return value is ignored.
      * stack.
@@ -2325,14 +2325,22 @@
      *   Stack: callee, this, args[0], ..., args[argc-1] => rval
      *   nuses: (argc+2)
      */ \
-    macro(JSOP_CALL_IGNORES_RV, 231, "call-ignores-rv", NULL, 3, -1, 1, JOF_UINT16|JOF_INVOKE|JOF_TYPESET)
-
+    macro(JSOP_CALL_IGNORES_RV, 231, "call-ignores-rv", NULL, 3, -1, 1, JOF_UINT16|JOF_INVOKE|JOF_TYPESET) \
+    /*
+     * If the value on top of the stack is not null or undefined, jumps to a 32-bit offset from the
+     * current bytecode.
+     *
+     *   Category: Statements
+     *   Type: Jumps
+     *   Operands: int32_t offset
+     *   Stack: cond => cond
+     */ \
+    macro(JSOP_COALESCE, 232, "coalesce", NULL, 5, 1, 1, JOF_JUMP|JOF_DETECTING)
 /*
  * In certain circumstances it may be useful to "pad out" the opcode space to
  * a power of two.  Use this macro to do so.
  */
 #define FOR_EACH_TRAILING_UNUSED_OPCODE(macro) \
-    macro(232) \
     macro(233) \
     macro(234) \
     macro(235) \