[analyzer] Fix false negative on post-increment of uninitialized variable.

Summary:
Currently clang static analyzer does warn on:
```
int x;
x+=1;
x-=1;
x=x+1;
x=x-1;
```
But does warn on:
```
int x;
x++;
x--;
--x;
++x;
```

This differential should fix that.
Fixes https://bugs.llvm.org/show_bug.cgi?id=35419

Reviewers: dcoughlin, NoQ

Reviewed By: dcoughlin

Subscribers: NoQ, xazax.hun, szepet, cfe-commits, a.sidorin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D40463

llvm-svn: 319411