fix VM uint conversion size bug, stricter int gen on JS (#22150)

* fix VM uint conversion bug, stricter int gen on JS

fixes #19929

* fix float -> uint64 conversion too

* no need to mask to source type

* simpler diff with explanation, add test for described issue

tests/vm/ttouintconv.nim

@@ -75,3 +75,10 @@ macro foo2() =
 
 foo()
 foo2()
+
+block:
+  const neg5VM = block:
+    let x = -5'i8
+    uint64(x)
+  let y = -5'i8
+  doAssert uint64(y) == neg5VM