[llvm] [AMDGPU] SIWholeQuadMode: avoid execz effects in exact regions (PR #101157)

[Carl Ritson via llvm-commits]

perlfu wrote:

- Rebased on to pre-commit tests
- Tighten the condition for shortening exact regions - makes change more targeted and thus has essentially no impact on any existing shaders unimpacted by this issue

In the main we might possibly have some more missed EXECZ regions, but they can only be from WQM to exact transitions.
Strict WWM enables extra lanes for short sections, so will never be EXECZ.
Strict WQM can only be EXECZ if it was entered into from an existing EXECZ region, so in principle this might be possible.
This would have to be full shader WQM -> exact -> strict WQM -- which is low probability.

Again I think this is probably best addressed after control flow lowering is reworked (i.e. wave transform).

https://github.com/llvm/llvm-project/pull/101157