In certain cases (i.e. long double on x86), the bit with we get from the
floating point semantics is different than the type size we compute for
the BitCast instruction. Pass this along to DoBitCast, so in there we
can check only the relevant bits for being initialized.
This also fixes a weirdness we still had in DoBitCast.
This reverts commit 54db16221c92eb52efbea90ad5b5d2a1d00cda3e.
Check for existence of __SIZOEF_INT128__ so we don't run those
tests on targets that don't have int128.
