When code size is the priority (Oz, MinSize attribute), help llvm

When code size is the priority (Oz, MinSize attribute), help llvm
turning a code like this:

if (foo)
   free(foo)

into that:
free(foo)

Move a call to free from basic block FB into FB's predecessor, P,
when the path from P to FB is taken only if the argument of free is
not equal to NULL.

Some restrictions apply on P and FB to be sure that this code motion
is profitable. Namely:
1. FB must have only one predecessor P.
2. FB must contain only the call to free plus an unconditional
   branch to S.
3. P's successors are FB and S.

Because of 1., we will not increase the code size when moving the call
to free from FB to P.
Because of 2., FB will be empty after the move.
Because of 2. and 3., P's branch instruction becomes useless, so as FB
(simplifycfg will do the job).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171762 91177308-0d34-0410-b5e6-96231b3b80d8