The change replaces 'set bit enumeration' with arithmetic
and bitwise operations. For example, the formula
'(((x & -x) + x) & x) ^ x' can be used to find the rightmost
contiguous bit mask. This is essentially the operation that
SetBitEnumerator.findNext() performed.
Removing this functionality reduces the complexity of the
ClusteredBitVector (a.k.a. SpareBitVector) implementation and,
more importantly, API which will make it easier to modify
the implementation of spare bit masks going forward. My end
goal being to make spare bit operations work more reliably on
big endian systems.
Side note:
This change modifies the emit gather/scatter functions so that
they work with an APInt, rather than a SpareBitVector, which
makes these functions a bit more generic. These functions emit
instructions that are essentially equivalent to the parallel bit
extract/deposit (PEXT and PDEP) instructions in BMI2 on x86_64
(although we don't emit those directly currently). They also map
well to bitwise manipulation instructions on other platforms (e.g.
RISBG on IBM Z). So we might find uses for them outside spare bit
manipulation in the future.
