[llvm] [IR] Use SmallVector::append to replace a loop. (PR #143586)

[Craig Topper via llvm-commits]

topperc wrote:

I just realized the ConstantAggregateZero part of this is dead code due to an earlier check for  ConstantAggregateZero. I'll just refactor this to handle undef earlier and remove the scalable check.

https://github.com/llvm/llvm-project/pull/143586