Minor changes to allow preservation of post dominator tree through PHI
elimination pass.
Also remove duplicate retrieval of dominator tree analysis.

This is a speculative change to support reworking on passes in AMDGPU
backend.

Reverting because mis-compiles:
- https://github.com/llvm/llvm-project/pull/131837
- https://github.com/llvm/llvm-project/pull/146320
- https://github.com/llvm/llvm-project/pull/146337

Follow up to the second bug that
https://github.com/llvm/llvm-project/pull/131837 introduced, described
in
https://github.com/llvm/llvm-project/pull/131837#issuecomment-3018777795.

PR which introduced the bug:
https://github.com/llvm/llvm-project/pull/131837.
Fixes a crash around dead registers which started in f5c62ee0fa04 by
verifying that the reused incoming register is also virtual.

The insertion point of COPY isn't always optimal and could eventually
lead to a worse block layout, see the regression test in the first
commit.

This change affects many architectures but the amount of total
instructions in the test cases seems too be slightly lower.

Add per-property has<Prop>/set<Prop>/reset<Prop> functions to
MachineFunctionProperties.

This relands commit #115111.
Use traditional way to update post dominator tree, i.e. break critical
edge splitting into insert, insert, delete sequence.
When splitting critical edges, the post dominator tree may change its
root node, and `setNewRoot` only works in normal dominator tree...
See

https://github.com/llvm/llvm-project/blob/6c7e5827eda26990e872eb7c3f0d7866ee3c3171/llvm/include/llvm/Support/GenericDomTree.h#L684-L687

Reverts llvm/llvm-project#115111 Causes #119511

Support critical edge splitting in dominator tree updater. Continue the
work in #100856.

Compile time check:
https://llvm-compile-time-tracker.com/compare.php?from=87c35d782795b54911b3e3a91a5b738d4d870e55&to=42b3e5623a9ab4c3648564dc0926b36f3b438a3a&stat=instructions%3Au