[llvm] [GlobalISel] avoid G_TRUNC with floating-point G_MERGE_VALUES source (PR #206733)

[Nathan Corbyn via llvm-commits]

https://github.com/cofibrant approved this pull request.

Looks good to me! Just a few nits. Let's wait for others to come back to this before merging, though.

https://github.com/llvm/llvm-project/pull/206733