Run LLVM coverage instrumentation passes before optimization passes

This matches the behavior of Clang and allows us to remove several
hacks which were needed to ensure functions weren't optimized away
before reaching the instrumentation pass.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -548,6 +548,11 @@ pub(crate) unsafe fn optimize(
                     llvm::LLVMRustAddPass(fpm, find_pass("lint").unwrap());
                     continue;
                 }
+                if pass_name == "insert-gcov-profiling" || pass_name == "instrprof" {
+                    // Instrumentation should be inserted before optimization.
+                    llvm::LLVMRustAddPass(mpm, find_pass(pass_name).unwrap());
+                    continue;
+                }
 
                 if let Some(pass) = find_pass(pass_name) {
                     extra_passes.push(pass);