[Lanai] Fix useDeprecatedPositionallyEncodedOperands errors.

This is a follow-on to https://reviews.llvm.org/D134073.

Lanai was almost clean: the only issue is that 'bit' behaves
differently than 'bits<1>', because only the 'bits' type preserves
unresolved references via 'keepUnsetBits()' in
TableGen/Record.h. Thus, use bits instead.

This issue _would_ have caused invalid instruction emission/decoding,
except that the PQ bits were being overriden after the fact by code in
'adjustPqBits' in MCTargetDesc/LanaiMCCodeEmitter.cpp, and
'PostOperandDecodeAdjust' in Disassembler/LanaiDisassembler.cpp.

Differential Revision: https://reviews.llvm.org/D134075