[RISCV] Select disjoint_or+not as xnor. (#147636)
A disjoint OR can be converted to XOR. And a XOR+NOT is XNOR. Idea
taken from #147279.
I changed the existing xnor pattern to have the not on the outside
instead of the inside. These are equivalent for xor since xor is
associative. Tablegen was already generating multiple variants
of the isel pattern using associativity.
There are some issues here. The disjoint flag isn't preserved
through type legalization. I was hoping we could recover it
manually for the masked merge cases, but that doesn't work either.
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