[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