mirflow, mir2cg: use dedicated statement kind for 'continue'

The statement is a terminator too, so the block folding and continuation
inlining needs to know also needs to know about it.

Author: zerbina

compiler/backend/mir2cg.nim

@@ -2814,8 +2814,6 @@ proc stmtToCgir(c; env; tree; n; stmts; bu) =
   of mnkRaise:
     c.emitLineTrace(env, c.prc.body.source[tree[n].info].info, stmts, bu)
     c.emitRaise(env, tree[tree.last(n)], stmts, bu)
-  of mnkContinue:
-    c.emitRaise(env, tree[tree.last(n)], stmts, bu)
   of mnkEmit, mnkAsm:
     c.emitLineTrace(env, c.prc.body.source[tree[n].info].info, stmts, bu)
     stmts.add c.emitToCgir(env, tree, n, bu)
@@ -3058,6 +3056,8 @@ proc toTree(c; env; tree; list: seq[Stmt], i: int, stmts, bu) =
   of Raise:
     c.useSourceLoc(tree[list[i].n].info, bu)
     c.emitRaise(env, tree[tree.last(list[i].n)], stmts, bu)
+  of Continue:
+    c.emitRaise(env, tree[tree.last(list[i].n)], stmts, bu)
   of Dispatch:
     c.useSourceLoc(tree[list[i].n].info, bu)
     if isSimpleCase(tree, list[i].n):

compiler/backend/mirflow.nim

@@ -16,6 +16,7 @@ type
     Break
     Return
     Raise
+    Continue
     Stmts
     Scope
     Block
@@ -41,8 +42,9 @@ using
 
 const
   withSub = {Block, Loop, Scope, If, Dispatch, Target, Try}
-  terminators = {Loop, Dispatch, Break, Return, Raise}
-    ## stmts (block-like and not) that act as control-flow terminators
+  terminators = {Loop, Dispatch, Break, Return, Raise, Continue}
+    ## statements (both block-like and not) that act as control-
+    ## flow terminators
 
 iterator statements(tree): (int, NodePosition) =
   ## Iterates over all MIR statement in order of appearance.
@@ -249,7 +251,7 @@ proc toStructured*(tree): seq[Stmt] =
     of mnkContinue:
       if tree[it, 0].kind != mnkUnwind:
         insertTry(tree.child(it, 0), labels[tree[it, 0].label])
-      append(Stmt(kind: Stmts, n: it))
+      append(Stmt(kind: Continue, n: it))
     of mnkDef, mnkDefCursor, mnkAsgn, mnkInit, mnkSwitch, mnkVoid:
       let e = tree.last(it)
       if tree[e].kind == mnkCheckedCall: