[llvm] [GlobalISel] Bail out of G_EXTRACT_VECTOR_ELT value tracking on width mismatch (PR #223554)

[Joel Walker via llvm-commits]

Joel-Wwalker wrote:

Pushed a second commit and retitled. While writing this up I found the same width blind spot in the known-bits and never-zero cases for the opcode, and the never-zero one gives a wrong answer: an i8 extracted from a `<2 x i32>` holding 256 is reported never zero, although it truncates to zero. Both are covered here now instead of in a follow-up; the description has the details.

Assisted by Claude (Anthropic).


https://github.com/llvm/llvm-project/pull/223554