fully exploited the dropped support of LLVM 8

This commit grepped for LLVM_VERSION_GE, LLVM_VERSION_LT, get_major_version and
min-llvm-version and statically evaluated every expression possible
(and sensible) assuming that the LLVM version is >=9 now

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
 }