Micro-optimize `InstSimplify`'s `simplify_primitive_clone`
r? ````@compiler-errors```` , since you already did #139411 and got randomly selected for #139638 (feel free to reassign!)
Another one similar in spirit to #139411, but this time for `simplify_primitive_clone`, which is doing a bit of redundant work. Might not show up in benches, but probably worth micro-optimizing since the transformation is run even for debug builds.
See inline comments for my reasoning for making these changes.