Prepare for new pass manager version of `MachineDominatorTreeAnalysis`.
We may need a machine dominator tree version of `DomTreeUpdater` to
handle `SplitCriticalEdge` in some CodeGen passes.

This patch make `LiveDebugVariables` can be used by passes outside of
`lib/CodeGen`.

If we run a pass that occurs between the split register allocation pass
without preserving this pass, it will be freed and recomputed until it
encounters the next pass that needs LiveDebugVariables.

However, `LiveDebugVariables` will raise an assertion due to the pass
being freed without emitting a debug value.

This is reason we need `LiveDebugVariables` to be available for passes
outside of lib/Codegen.

Explicitly include some headers or forward-declare types, in
preparation for removing an include that pulls in many transitive
headers.

Use isPhysical/isVirtual methods.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D141715

Prior to this patch, variadic DIExpressions (i.e. ones that contain
DW_OP_LLVM_arg) could only be created by salvaging debug values to create
stack value expressions, resulting in a DBG_VALUE_LIST being created. As of
the previous patch in this patch stack, DBG_INSTR_REF's syntax has been
changed to match DBG_VALUE_LIST in preparation for supporting variadic
expressions. This patch adds some minor changes needed to allow variadic
expressions that aren't stack values to exist, and allows variadic expressions
that are trivially reduceable to non-variadic expressions to be handled
similarly to non-variadic expressions.

Reviewed by: jmorse

Differential Revision: https://reviews.llvm.org/D133926

This patch makes two notable changes to the MIR debug info representation,
which result in different MIR output but identical final DWARF output (NFC
w.r.t. the full compilation). The two changes are:

  * The introduction of a new MachineOperand type, MO_DbgInstrRef, which
    consists of two unsigned numbers that are used to index an instruction
    and an output operand within that instruction, having a meaning
    identical to first two operands of the current DBG_INSTR_REF
    instruction. This operand is only used in DBG_INSTR_REF (see below).
  * A change in syntax for the DBG_INSTR_REF instruction, shuffling the
    operands to make it resemble DBG_VALUE_LIST instead of DBG_VALUE,
    and replacing the first two operands with a single MO_DbgInstrRef-type
    operand.

This patch is the first of a set that will allow DBG_INSTR_REF
instructions to refer to multiple machine locations in the same manner
as DBG_VALUE_LIST.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D129372

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716