[llvm] [DebugInfo][DWARF] Set is_stmt on first non-line-0 instruction in BB (PR #105524)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Had a free moment so updated this - the previous version (as noted in my response to @pogo59 above) would still merge adjacent DLs across BB boundaries if they were exactly identical, rather than differing in column number; I've changed this to now only merge adjacent DLs within a BB, which strictly reduces the cases where we'll merge identical DLs. 

https://github.com/llvm/llvm-project/pull/105524