Fixes #5034 illformed AST from getImpl with proc returning value (#17976)

* Fixes 5034

* address comments

tests/macros/tmacrogetimpl.nim

@@ -0,0 +1,31 @@
+import macros
+
+# bug #5034
+
+macro copyImpl(srsProc: typed, toSym: untyped) =
+  result = copyNimTree(getImplTransformed(srsProc))
+  result[0] = ident $toSym.toStrLit()
+
+proc foo1(x: float, one: bool = true): float =
+  if one:
+    return 1'f
+  result = x
+
+proc bar1(what: string): string =
+  ## be a little more adversarial with `skResult`
+  proc buzz: string =
+    result = "lightyear"
+  if what == "buzz":
+    result = "buzz " & buzz()
+  else:
+    result = what
+  return result
+
+copyImpl(foo1, foo2)
+doAssert foo1(1'f) == 1.0
+doAssert foo2(10.0, false) == 10.0
+doAssert foo2(10.0) == 1.0
+
+copyImpl(bar1, bar2)
+doAssert bar1("buzz") == "buzz lightyear"
+doAssert bar1("macros") == "macros"