Fixed call argument value snapshotting.

[xeioex]

Since fd5e523 (0.9.7), njs has evaluated all call argument expressions before emitting the frame and PUT_ARG instructions, this fixed await expressions in call arguments. This preserved argument side effects before callee validation, but left PUT_ARG reading argument values only after later argument expressions had already run.

As a result, an earlier argument backed by a mutable variable slot could observe a later mutation of the same slot. For example, f(a, a = 2) passed 2 as both arguments instead of preserving the first value as 1. The same issue applied to later argument effects such as getters.

The fix is to preserve arguments using temporary registers where appropriate.

This fixes #1059 issue on Github.

[VadimZhestikov]

1. Observation: njs_generate_argument_uses_index has dead recursive tree-walk logic (children are always
   NULL in current call sites). A comment explaining that this function is called only on leaf nodes would
   prevent future confusion — or the child checks could be removed with a njs_assert(node->left == NULL &&
   node->right == NULL).

2. Suggestion: The node->left->index == index pre-check before recursion in
   njs_generate_argument_uses_index would silently fail to detect variable use inside compound subtrees if
   the function's call conditions ever change. Consider replacing with an unconditional recursive call
   (which node->temporary would likely prevent from being reached in practice anyway).

Both are minor.

[VadimZhestikov]

Looks good

[xeioex]

Observation: njs_generate_argument_uses_index has dead recursive tree-walk logic (children are always
NULL in current call sites)

The recursive walk is not dead. This is easily discoverable with simple assert statement + current tests.

It is reached by compound argument expressions whose result aliases a mutable slot, for example assignment expressions f(a = b, b = 2) through left, and comma expressions f(a, (a = 2, a), a = 3) through right.