Rollup merge of #78848 - DevJPM:ci-llvm-9, r=nikic

Bump minimal supported LLVM version to 9

This bumps the minimal tested llvm version to 9.
This should enable supporting newer LLVM features (and CPU extensions).

This was motived by #78361 having to drop features because of LLVM 8 not supporting certain CPU extensions yet.
This was declared relatively uncontroversial on [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Min.20Supported.20LLVM.20Upgrade.20Process.3F/near/215957859).

Paging ````@eddyb```` because there was a comment in the [dockerfile](https://github.com/rust-lang/rust/blob/master/src/ci/docker/host-x86_64/x86_64-gnu-llvm-8/Dockerfile#L42) describing a hack (which I don't quite understand) which was also blocked by not having LLVM 9.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -377,11 +377,6 @@ fn get_pgo_use_path(config: &ModuleConfig) -> Option<CString> {
 }
 
 pub(crate) fn should_use_new_llvm_pass_manager(config: &ModuleConfig) -> bool {
-    // We only support the new pass manager starting with LLVM 9.
-    if llvm_util::get_major_version() < 9 {
-        return false;
-    }
-
     // The new pass manager is disabled by default.
     config.new_llvm_pass_manager
 }