Ping: PR 63340: Avoid harmful union classes in ira-costs.c

[Richard Sandiford]

Two-patch IRA series to fix PR 63340:

https://gcc.gnu.org/ml/gcc-patches/2014-09/msg02636.html
https://gcc.gnu.org/ml/gcc-patches/2014-09/msg02637.html

It also fixes PR 63403.

Thanks,
Richard