Actually, more importantly, texkill _does_ operate on all four components for 2.0, and for 1.x it has very restricted usage anyway (you can't use it on arbitrary expressions). Not sure why the test is failing in that case...
You are totally right, there is a reason why the test is failing. Currently we are failing to translate texkill to spir-v correctly, it is just taking the first component into account. I missed setting up a src swizzle in vsir_program_lower_texkills().
For some reason I thought that the reason why the test was failing in native was because texkill only considered the first 3 components, but this was a ir.c problem.
I replaced the patch that introduced JUMP_TEXKILL.