Previously we used a fixed assembly string as well as encoding for the
carry-in vector mask, since it will always be there. However, this makes
both AsmParser and disassembler to either create a garbage MCOperand for
the mask or fail to add one as a whole. This wouldn't be a problem for
majority of the cases but tools like llvm-mca who relies on MCInst will
fail to account for the register dependency on these mask operands.
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