A merge block must be dominated by its own header.
This CL checks the cases where that fails because the
merge block is also the:
- the default or case header for a switch (a different header)
- the true-head, false-head, or premerge-head for an if-selection
with a different header
Bug: tint:3
Change-Id: I6dd1fae162e9d33bd9a0b43d3ca9558cebed058b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/22680
Reviewed-by: dan sinclair <dsinclair@google.com>
bdcd04b1a2