Backport UB workaround for GCC LTO builds

[thesamesam]

See #57740 and #24952.

Without this, LLVM is miscompiled (because of UB in User.cpp) when built with GCC LTO. Given that -fno-lifetime-dse is a GCC only flag and we know it was miscompiled otherwise with GCC LTO, I think we can safely backport it.

Needs 626849c and 47f5c54.

[llvmbot]

/branch llvm/llvm-project-release-prs/issue62699

[llvmbot]

/pull-request llvm/llvm-project-release-prs#445

[thesamesam]

/cherry-pick ce990b5

[llvmbot]

/branch llvm/llvm-project-release-prs/issue62699