Analyses should be marked as analyses.

Otherwise they are prone to get ignored by the legacy analysis cache mechanism and get scheduled redundantly.

The existing analysis was already a pimpl wrapper.

I have extracted legacy pass logic to a LDVImpl wrapper named
`LiveDebugVariables` which is the analysis::Result now. This controls
whether to activate the LDV (depending on `-live-debug-variables` and
DIsubprogram) itself.

The legacy and new analysis only construct the LiveDebugVariables.

VirtRegRewriter will test this.

LDV could reorder reinserted fragment and non-fragment debug values for
the same variable (compared to the input order), potentially resulting
in stale values being presented.

For example, before:

  DBG_VALUE 1001, $noreg, !13, !DIExpression(DW_OP_LLVM_fragment, 0, 16)
  DBG_VALUE 1002, $noreg, !13, !DIExpression(DW_OP_LLVM_fragment, 16, 16)
  DBG_VALUE %0, $noreg, !13, !DIExpression()

After (without this patch):

  DBG_VALUE %stack.0, 0, !13, !DIExpression()
  DBG_VALUE 1002, $noreg, !13, !DIExpression(DW_OP_LLVM_fragment, 16, 16)
  DBG_VALUE 1001, $noreg, !13, !DIExpression(DW_OP_LLVM_fragment, 0, 16)

It would also reorder DBG_VALUEs for different variables. Although that
does not matter for the debug information output, it resulted in some
noise in before/after pass diffs.

This should hopefully align so that instruction referencing and
DBG_VALUE emit debug instructions in the same order (see the
sdag-salvage-add.ll change).

These would implicitly cast the register to `unsigned`. Switch most of
them to use printReg will give a more readable output. Change some
others to use Register::id() so we can eventually remove the implicit
cast to `unsigned`.

Use hasPhys instead of MCRegister::isPhysicalRegister.

I think the MCRegister returned from getPhys can only contain a physical
register or 0. hasPhys checks that the register returned from getPhys is non-zero.
So I think they are equivalent in this usage.

- Add `LiveIntervalsAnalysis`.
- Add `LiveIntervalsPrinterPass`.
- Use `LiveIntervalsWrapperPass` in legacy pass manager.
- Use `std::unique_ptr` instead of raw pointer for `LICalc`, so
destructor and default move constructor can handle it correctly.

This would be the last analysis required by `PHIElimination`.

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.