IEqual32 and INotEqual32 against a constant fold when the other
operand has a value set of at most 64 values (built through shifts,
bitfield extracts, and/or/xor/mul/umin with a constant, add and
select) that excludes the constant or holds only it, or when a known
bit of the operand contradicts the constant. Known bits come from
constants, shifts and bitfield extracts by constants, and/or/xor, add
(up to the first unknown bit), mul (trailing zeros), umin, select, phi
and lane reads. The value set of x & mask leaves out the known bits of
x. ULessThan32 against a constant folds from the known-bit and
value-set bounds.
A fold pass computes the known bits and value set of each compared
operand once, so a chain of compares against one operand does not
repeat the analysis, and a sum is not enumerated when its two operand
sets hold more than 65 values together, since without wraparound it
then has more than 64 values.
All folds are identities of the operand's possible values; out-of-range
M0 handling of the relative indexing lowerings is unchanged. With
M0 = 4i + k, three of every four entries of an s_movrels/s_movreld
compare chain fold away.
