[CodeGen] Fix for PR39094.
When using MachineInstr to get SlotIndex, the MI could not be a debug
instruction. mi2iMap does not contain debug instructions in it.
After enabling DBG_LABEL in the generated code, the first instruction in
the bundle may be a debug instruction. In this patch, I use the first
non-debug instruction in the bundle to query SlotIndex in mi2iMap.
Bugzilla report: https://bugs.llvm.org/show_bug.cgi?id=39094
Differential revision: https://reviews.llvm.org/D52927
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344770
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