[InstCombine] Preserve !annotation on newly created instructions.

If the source instruction has !annotation metadata, all instructions
created during combining should also have it. Tell the builder to
add it.

The !annotation system was discussed on llvm-dev as part of
'RFC: Combining Annotation Metadata and Remarks'
(http://lists.llvm.org/pipermail/llvm-dev/2020-November/146393.html)

This patch is based on an earlier patch by Francis Visoiu Mistrih.

Reviewed By: thegameg, lebedev.ri

Differential Revision: https://reviews.llvm.org/D91444