Ensure forward progress on trace exit to BC_ITERN.

Also use a safer way to force a static dispatch for BC_RET*.
Reported by Bartel Eerdekens. Analyzed by Peter Cawley. #1000 #1045

src/vm_x64.dasc

@@ -2453,7 +2453,7 @@ static void build_subroutines(BuildCtx *ctx)
   |  mov r12, [RA]
   |  mov rsp, RA			// Reposition stack to C frame.
   |.endif
-  |  test RDd, RDd; js >9		// Check for error from exit.
+  |  cmp RDd, -LUA_ERRERR; jae >9	// Check for error from exit.
   |  mov L:RB, SAVE_L
   |  mov MULTRES, RDd
   |  mov LFUNC:KBASE, [BASE-16]
@@ -2469,6 +2469,8 @@ static void build_subroutines(BuildCtx *ctx)
   |  movzx OP, RCL
   |  add PC, 4
   |  shr RCd, 16
+  |  cmp MULTRES, -17			// Static dispatch?
+  |  je >5
   |  cmp OP, BC_FUNCF			// Function header?
   |  jb >3
   |  cmp OP, BC_FUNCC+2			// Fast function?
@@ -2491,6 +2493,15 @@ static void build_subroutines(BuildCtx *ctx)
   |  mov KBASE, [KBASE+PC2PROTO(k)]
   |  jmp <2
   |
+  |5:  // Dispatch to static entry of original ins replaced by BC_JLOOP.
+  |  mov RA, [DISPATCH+DISPATCH_J(trace)]
+  |  mov TRACE:RA, [RA+RD*8]
+  |  mov RCd, TRACE:RA->startins
+  |  movzx RAd, RCH
+  |  movzx OP, RCL
+  |  shr RCd, 16
+  |  jmp aword [DISPATCH+OP*8+GG_DISP2STATIC]
+  |
   |9:  // Rethrow error from the right C frame.
   |  mov CARG2d, RDd
   |  mov CARG1, L:RB