LLVM 18 cannot read llvm-ir produced by rustc 1.72.1

[glandium]

This is a regression from 6251134.

05b86a8 addressed a part of the regression. For instance, this:

  call void @llvm.dbg.value(metadata i64 or (i64 zext (i32 ptrtoint (ptr @anon.436463c58f189989edb51bc04335cb6d.21 to i32) to i64), i64 shl (i64 zext (i32 trunc (i64 lshr (i64 ptrtoint (ptr @anon.436463c58f189989edb51bc04335cb6d.21 to i64), i64 32) to i32) to i64), i64 32)), metadata !4453, metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64)), !dbg !4493

used to cause an error but doesn't anymore with that fix.

What still causes an error, though, is e.g.:

  call void @llvm.dbg.value(metadata ptr inttoptr (i64 or (i64 zext (i32 ptrtoint (ptr @anon.202156da7d09c46fe0b29195d5b08186.240 to i32) to i64), i64 shl (i64 zext (i32 trunc (i64 lshr (i64 ptrtoint (ptr @anon.202156da7d09c46fe0b29195d5b08186.240 to i64), i64 32) to i32) to i64), i64 32)) to ptr), metadata !14853, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64)), !dbg !14944

from this object file: https://drive.google.com/file/d/17fb-_f363yRFibtkJjRQ5qihnAuOPbdc/view?usp=sharing

Cc: @nikic

[nikic]

Reduced input: https://gist.github.com/nikic/e2a95cbbc2d9d21b1bea4912147fce75

The fact that the same constant expression is used by metadata in both functions seems to be relevant. Possibly in that case the metadata gets encoded globally rather than per-function :/

[nikic]

Okay, I think I'm going to approach this from the other direction: Pretend that the constant has been deleted, which means the ValueAsMetadata reference will be replaced with UndefValue.

[glandium]

FYI, there is no remaining error building Firefox with cross-language LTO after 236228f