malloc() always compare different

[nunoplopes]

see this:

define i1 @two_nonnull_mallocs() {
%0:
  %m = malloc i64 4
  %n = malloc i64 4
  %cmp = icmp eq * %m, %n
  ret i1 %cmp
}
=>
define i1 @two_nonnull_mallocs() {
%0:
  ret i1 0
}
Transformation seems to be correct!

We ignore the fact that malloc() may return null. LLVM seems to be going into a stricter direction. We need to reevaluate and test if a stricter memory model & refinement criteria makes sense now.

[nunoplopes]

Related with #732

[aqjune]

For this case only, tgt has no allocation - so we can individually assign nonnull-ness to each malloc.

To correctly deal with this issue, I think we need a mapping between allocations in src and tgt.
For the mapped blocks, the nonnull-ness must be equivalent.