[LegalizeTypes] Only expand CTLZ/CTTZ/CTPOP during type promotion if the new type is legal.

We might be promoting a large non-power of 2 type and the new type
may need to be split. Once we split it we may have a ctlz/cttz/ctpop
instruction for the split type.

I'm also concerned that we may create large shifts with shift amounts
that are too small.