[Bug tree-optimization/100112] missed optimization for dead code elimination at -O3, -Os (vs. -O1, -O2)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100112

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Oh, and walking doesn't help since we may not use TBAA for the lookup but the
load handling did use TBAA and only because of that figured a better VUSE to
record the reference into the hashtables.

A smaller testcase is

int *c, *b;
void foo()
{
  int *tem = b;
  *tem = 0;
  tem = c;
  c = tem;
}

so it really boils down to last_vuse being good and bad at the same time.