The lowerKillI1 method wrongly handled cases where it inserted a
new S_BRANCH instruction when the kill was not the only terminator,
and then tried to split the block.

`SI_KILL_I1_TERMINATOR -1,0` doesn't have any effect. Instead of
lowering to an unconditional branch, we remove the instruction and
insert an unconditional branch only if the instruction is the last
terminator. No split is needed in this case (if the last terminator
has been reached, then the whole block was processed).

Also stop generating an unconditional branch in splitBlock: this
branch was redundant since TermMI is promoted to a
terminator that fallsthrough to the next block already.

Solves SWDEV-508819

Ensure shaders running WQM (PS) enter at the earliest point irrespective
of WQM marking.

Replace S_XOR with S_ANDN2 when computing the kill mask in demote/kill
lowering. This has the effect of AND'ing demote/kill condition with exec
which is needed for proper live mask update.

The S_XOR is inadequate because it may return true for lane with exec=0.

This patch fixes an image corruption in game.

I think the issue went unnoticed because demote/kill condition is often
naturally dependent on exec, so AND'ing with exec is usually not
required.

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