[llvm] [GISel] Teach computeKnownBitsImpl to handle COPY instructions that change bit width. (PR #118924)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I haven't reduced a test case yet, but wanted to make sure this is the right fix.

Easier to judge this with the test but probably

https://github.com/llvm/llvm-project/pull/118924