(expand_float): When converting an unsigned type U to a smaller or equal-sized floati...

(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...

(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.

From-SVN: r3778