[InstCombine] Teach the move free before null test opti how to deal with noop casts

[InstCombine] Teach the move free before null test opti how to deal with noop casts

InstCombine features an optimization that essentially replaces:
if (a)
  free(a)
into:
free(a)

Right now, this optimization is gated by the minsize attribute and therefore
we only perform it if we can prove that we are going to be able to eliminate
the branch and the destination block.

However when casts are involved the optimization would fail to apply, because
the optimization was not smart enough to realize that it is possible to also
move the casts away from the destination block and that is harmless to the
performance since they are just noops.
E.g.,
foo(int *a)
if (a)
  free((char*)a)

Wouldn't be optimized by instcombine, because
- We would refuse to hoist the `bitcast i32* %a to i8` in the source block
- We would fail to see that `bitcast i32* %a to i8` and %a are the same value.

This patch fixes both these problems:
- It teaches the pattern matching of the comparison how to look
  through casts.
- It checks that whether the additional instruction in the destination block
  can be hoisted and are harmless performance-wise.
- It hoists all the code of the destination block in the source block.

Differential Revision: D53356

llvm-svn: 345644