Remove the branch merging optimisations for slice patterns

They were only correct in the simplest case. Some of the optimisations
are certainly possible but should be introduced carefully and only
when the whole pattern codegen infrastructure is in a better shape.

Fixes #16648.
auto merge of #16883 : jakub-/rust/issue-16648, r=pcwalton

They were only correct in the simplest case. Some of the optimisations
are certainly possible but should be introduced carefully and only
when the whole pattern codegen infrastructure is in a better shape.

Fixes #16648.