bitv: Avoid undefined behavior in mask_for_bits

It was relying on the fact that x86 shifts are effectively %BITS, so
shifting a u32 by 32 preserves all bits.  But this is undefined behavior
in general.  Add an explicit %BITS to be safe.
If codegen is fine, I'm happy with leaving it like this for semantic value. multiple modulo stuff hurts my brain, for the most part.