[llvm] [AArch64][GlobalISel] Ignore debug uses in mul-constant combine (PR #228100)

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GopinathBalaji wrote:

Thank you for the quick review.
@davemgreen I have changed `use_nodbg_instructions().begin()` to `use_instr_nodbg_begin()` as suggested.

@arsenm I have removed the new debug info and used `mir-debugify` as suggested. Two `RUN` commands are required in this case. The first command runs `mir-debugify` and writes the generated debugified MIR to %t.mir. The second command reads that MIR back in before running the post-legalizer combiner. 
I found that reparsing is necessary because it rebuilds the register use-list from the serialized instruction order, allowing the generated `DBG_VALUE` to precede the real user and reproduce the original bug. Finally I used `mir-strip-debug` to remove the synthetic debug information before comparing the output with `FileCheck`.

https://github.com/llvm/llvm-project/pull/228100