[llvm] [CodeGen] Port `GCLowering` to new pass manager (PR #75305)

[Matt Arsenault via llvm-commits]

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@@ -27,15 +27,22 @@
 
 using namespace llvm;
 
+/// Lower barriers out of existance (if the associated GCStrategy hasn't
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arsenm wrote:

spelling s/existance/existence/

https://github.com/llvm/llvm-project/pull/75305