(expand_float): When converting an unsigned type U

(expand_float): When converting an unsigned type U
to a smaller or equal-sized floating type F where there's no
hardware support for unsigned-to-float conversion, use a temporary
floating type larger than F if possible, since this avoids
rounding problems that are worst when U is just greater than
the maximum signed integer of the same size.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@3778 138bc75d-0d04-0410-961f-82ee72b054a4