Bug 1545540 - Part 1: Use BytecodeSection.lastTarget_=-1 to mark last opcode is not jump target. r=jorendorff

Previously the initial value of BytecodeSection.lastTarget_ is chosen to make
the following condition true:
  lastTargetOffset() + ptrdiff_t(JSOP_JUMPTARGET_LENGTH) == -1

BytecodeOffset class introcuced in part 3 requires the offset value to be
either -1 (invalid) or non-negative, and that rule conflicts with above.

Changed the initial value of BytecodeSection.lastTarget_ to -1 and added
the check for that condition in BytecodeSection::lastOpcodeIsJumpTarget,
so that we don't use negative value in BytecodeOffset.

Differential Revision: https://phabricator.services.mozilla.com/D33052

--HG--
extra : moz-landing-system : lando

js/src/frontend/BytecodeSection.h

@@ -139,7 +139,8 @@ class BytecodeSection {
 
   // Check if the last emitted opcode is a jump target.
   bool lastOpcodeIsJumpTarget() const {
-    return offset() - lastTarget_.offset == ptrdiff_t(JSOP_JUMPTARGET_LENGTH);
+    return lastTarget_.offset != -1 &&
+           offset() - lastTarget_.offset == ptrdiff_t(JSOP_JUMPTARGET_LENGTH);
   }
 
   // JumpTarget should not be part of the emitted statement, as they can be
@@ -239,7 +240,7 @@ class BytecodeSection {
   // ---- Jump ----
 
   // Last jump target emitted.
-  JumpTarget lastTarget_ = {-1 - ptrdiff_t(JSOP_JUMPTARGET_LENGTH)};
+  JumpTarget lastTarget_ = {-1};
 
   // ---- Stack ----