[thesyncim]

Stacked on #2; this PR's own changes.

The rewritten code adds names that start with an underscore, some of them declared before or inside the loop. It also calls len, min, max and copy, converts with uint64 and the element types, and refers to the packages simd and unsafe. The function, its package or an import can give any of those names another meaning, and then the vector loop silently computes something else. These all go wrong on master:

func F(dst, a []int32, _i int) {
	for i := 0; i < _i; i++ {
		dst[i] = a[i] + 1
	}
}

The vector loop's condition reads loopvec's _i instead of the parameter, so it never runs. In a package that declares func len(s []int32) int { return cap(s) / 4 }, the vector loop takes its length from that len and stops a quarter of the way. And a sum written as for n := range n { s += a[n] }, or as for n := n - 1; n >= 0; n--, finishes its last elements with for n := _i; n < n; n++, which never runs, so the result is short.

A loop that would otherwise be rewritten is now left alone, with its own reason, when:

• its function uses an identifier that starts with an underscore, other than _. Rather than track which names each part of the emitter uses, this covers the names loopvec adds now and any it adds later;
• one of the names the code relies on, looked up from the loop, resolves to something other than the predeclared one or the imported package. The lookup starts at the loop's position, so a max declared after the loop doesn't count. The overlap helper's own names (uintptr, int, bool, any, false) are checked at package level, where it is declared. The toolexec wrapper now records scopes for this;
• the loop itself declares one of those names, or a name its range operand or initial values use (for n := range n, for n := n - 1; ..., for len := range a).

These checks run last, so a loop that can't be vectorized anyway keeps the reason that matters.

The second commit fixes the same kind of bug in fixBlankParams. It renames _ parameters to _p0, _p1, ... to work around a compiler bug with simd code, but it did that in every function of a rewritten file, so func Offset(_ int) int { return _p0 } started returning its argument instead of a package-level _p0. Now it only renames in the functions loopvec rewrote, which can't mention such names.

No loop in std, gonum or goav1 stops being vectorized. reserved_names.txt and blank_params.txt cover each case. A rewrite that doesn't type-check still falls back in toolexec; toolexec_falls_back_when_rewrite_does_not_compile.txt used a parameter named simd to get one, which is now rejected earlier, so it uses a package that declares _loopvecOverlap itself instead.