Move the creation of OperandMap from createDagOperandMapping to the loop
in addDagOperandMapping. Expand it to store the DAG operand number and
the MI operand number which will be different when there are tied
operands.

Rename createDagOperandMapping to checkDagOperandMapping to better
describe the remaining code.

I didn't lift the restriction that a source instruction can only have
one tied operand, but we should be able to if we have a use case.

There's a slight difference in the generate output. We now check that
operand 0 and 2 of QC_MVEQI are equal instead of operand 1 and 2. This
should be equivalent since operand 0 and 1 have a tied constraint.

- Add various instruction subclass/sub-slice accessors to
`CodeGenTarget`.
- Delete unused `inst_begin` and `inst_end` iterators.
- Rename `Instructions` to `InstructionMap` and `getInstructions` to
`getInstructionMap` to better represent their meaning.
- Use these new accessors in InstrInfoEmitter

Add a convenience wrapper struct for the `bit_value_t` enum type to host
various constructors, query, and printing support. Also refactor related
code in several places. In `getBitsField`, use `llvm::append_range` and
`SmallVector::append()` and eliminate manual loops. Eliminate
`emitNameWithID` and instead use the `operator <<` that does the same
thing as this function. Have `BitValue::getValue()` (replacement for
`Value`) return std::optional<> instead of -1 for unset bits. Terminate
with a fatal error when a decoding conflict is encountered.

so that subclasses can provide the appropriate MCAsmInfo to print
MCExpr objects.

At present, llvm/utils/TableGen/AsmMatcherEmitter.cpp constucts a
generic MCAsmInfo.

Copy the whole struct instead of copying both fields.

We can use the MIOperandNo and NumMIOperands from the last operand
instead using a loop.

We were counting the number of tied operands in two different loops.

Work towards separating the ABI existence of libcalls vs. the
lowering selection. Set libcall selection through enums, rather
than through raw string names.