[Assignment Tracking][8/*] Add DIAssignID merging utilities

The Assignment Tracking debug-info feature is outlined in this RFC:

https://discourse.llvm.org/t/
rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir

Add method:

  Instruction::mergeDIAssignID(
      ArrayRef<const Instruction* > SourceInstructions)

which merges the DIAssignID metadata attachments on `SourceInstructions` and
`this` and replaces uses of the original IDs with the new shared one.

This is used when stores are merged, for example sinking stores out of a
if-diamond CFG or vectorizing contiguous stores.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133291

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