Reapply [InstCombine] Extract bitcast -> gep transform

Relative to the original patch, an InstCombine test has been
added to show a previously missed pattern, and the Coroutine
test that resulted in the revert has been regenerated.

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Move this into a separate function, to make sure that early
returns do not accidentally skip other transforms. This previously
happened for the isSized() check, which skipped folds like
distributing a bitcast over a select.