Manipulating the signaling NaN used for the hole and uninitialized double
field sentinel in C++, e.g. with bit_cast or HeapNumber::value()/set_value(),
will change its value on ia32 (the x87 stack is used to return values and
stores to the stack silently clear the signalling bit).

BUG=v8:5495

Review-Url: https://codereview.chromium.org/2652553003
Cr-Commit-Position: refs/heads/master@{#42609}