The MCOperandPredicate seems to allow symbols as well as immediates, but
the parser/matcher does not due to `isCLUIImm`. This brings both in line
with each other, and should prevent trying to compress a `lui` with a
symbol, which cannot be emitted as a `c.lui` as there are no relocations
for this as `R_RISCV_RVC_LUI` is deprecated/removed.
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