When we're lowering to a split sequence, we only need one
materialization of the zero constant. Our codegen looks something like
this:
vmv.v.i v24, 0
vmerge.vim v8, v24, -1, v0
vmv1r.v v0, v16
vmerge.vim v16, v24, -1, v0
Note: Doing this specific case since it was pointed out in
https://github.com/llvm/llvm-project/pull/110164#discussion_r1778268391,
but it's worth noting that we have the same basic problem (over costing
split operations with split invariant terms) at multiple places through
this file.
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