[llvm-branch-commits] [X86][ProfCheck] Synthesize profile information where relevant (PR #181578)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/181578

When creating loops to lower some AMX intrinsics, it is often the case
we have enough information to synthesize profile metadata for the latch.
This patch makes it so that we either set branch weights if everything
is a known constant, or set unknown weights if we do not have constants.