cmd/compile: do not invert loops that would overflow or underflow

On the final iteration we need space below start (which becomes end)
such that i-step does not overflow or underflow.

In other words the code used to assume that the last time the loop header
execute `start < i - step` (or `<=`, `>` `>=` based on the loop)
is always false.

And it seems correct since by definition the only way for it to be the
last's loop header execution is when the condition becomes false.

However here is an example with uint (even tho the code doesn't
already support them) to make things simpler:
  start = 1
  i = 2
  step = 100

We do 2 - 100 which should give us 1 < -98 == false breaking the loop;
Instead we get 18446744073709551518 which gives
1 < 18446744073709551518 == true which keeps the loop going.

This patch fixes this issue by ensuring that in the last execution of
a loop header the induction variable does not underflow or overflow.

Fixes #78303

Change-Id: I64e8e8592b023d79fdbc7f1598d584726ed601f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/758801
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Jakub Ciolek <jakub@ciolek.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions