Try to harden the recursive simplification still further. This is again

spotted by inspection, and I've crafted no test case that triggers it on
my machine, but some of the windows builders are hitting what looks like
memory corruption, so *something* is amiss here.

This patch takes a more generalized approach to eliminating
double-visits. Imagine code such as:

  %x = ...
  %y = add %x, 1
  %z = add %x, %y

You can imagine that if we simplify %x, we would add %y and %z to the
list. If the use-chain order happens to cause us to add them in reverse
order, we could pull %y off first, and simplify it, adding %z to the
list. We now have %z on the list twice, and will reference it after it
is deleted.

Currently, all my test cases happen to not trigger this, likely due to
the use-chain ordering, but there seems no guarantee that such
a situation could not occur, so we should handle it correctly.

Again, if anyone knows how to craft a testcase that actually triggers
this, please let me know.

llvm-svn: 153397