Unlike BitSel, which enforces a strict one-hot mask encoding, this type
permits any number of bits to be set or cleared. This is used to accumulate
selections for batched operations on a register in real memory.
R: The register element that this mask governs.
This must only be constructed by combining BitSel selection masks produced
through the accepted chains of custody beginning with BitIdx values.
Bit-masks not constructed in this manner are not guaranteed to be correct in the
caller’s context and may lead to incorrect memory behaviors.