Ensure live intervals for EXEC and SCC are removed on all paths which
generate instructions.

Create utility mechanism for finding wave size dependent opcodes used to
manipulate exec/lane masks.

This patch replaces SmallSet<T *, N> with SmallPtrSet<T *, N>.  Note
that SmallSet.h "redirects" SmallSet to SmallPtrSet for pointer
element types:

  template <typename PointeeType, unsigned N>
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};

We only have 140 instances that rely on this "redirection", with the
vast majority of them under llvm/. Since relying on the redirection
doesn't improve readability, this patch replaces SmallSet with
SmallPtrSet for pointer element types.

Change dominator tree updates to also handle post dominator tree.

That pass no longer exists, since
5df2af8b0ef33f48b1ee72bcd27bc609b898da52 has merged it into
SIPreEmitPeephole.

- 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`.

- Add `SlotIndexesAnalysis`.
- Add `SlotIndexesPrinterPass`.
- Use `SlotIndexesWrapperPass` in legacy pass.