Closes https://github.com/llvm/llvm-project/issues/99116
Implements `firstbitlow` by extracting common functionality from
`firstbithigh` into a shared function while also fixing a bug for an edge
case where `u64x3` and larger vectors will attempt to create vectors
larger than the SPRIV max of 4.
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Co-authored-by: Steven Perron <stevenperron@google.com>
