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 is a subset of #68288, with hopefully narrower scope. It does not
support bitcasting to non-integral types yet.
Bitfields are supported, but only if they result in a full byte-sized
final buffer. It does not support casting from null-pointers yet or
casting from indeterminate bits.
The tests are from #68288 and partially from #74775.
The `BitcastBuffer` struct is currently always working in single bits,
but I plan to (try to) optimize this for the common full-byte case.
