False positive "uninitialized read" on bitfield access

This happened while running Blink (the rendering engine for Chrome) tests.

First reported here:
https://code.google.com/p/chromium/issues/detail?id=498519

The code was essentially the following code.

``` c++
class ElementRareData {
public:
ElementRareData() : m_proxyCount(0) { }
uint32_t proxyCount() const { return m_proxyCount; }
...
private:
unsigned short m_proxyCount: 10;
...
};

ElementRareData erd;
uint32_t x = erd.proxyCount(); // (*)
```

`m_proxyCount` does not have neighboring bits used by other bitfields, so the compiler
adds padding of 6 bits. On the (*) timing, compiler emits 16bit read instruction and
applies 10bit bitmask, then extend to unsigned 32bit, but at the read instruction
it shouldn't be counted as "uninitialized read".