rustc_llvm: Add a -Z print-llvm-stats option to expose LLVM statistics.

LLVM has a neat [statistics] feature that tracks how often optimizations kick
in. It's very handy for optimization work. Since we expose the LLVM pass
timings, I thought it made sense to expose the LLVM statistics too.

[statistics]: https://llvm.org/docs/ProgrammersManual.html#the-statistic-class-stats-option

compiler/rustc_codegen_llvm/src/lib.rs

@@ -181,6 +181,11 @@ impl WriteBackendMethods for LlvmCodegenBackend {
             llvm::LLVMRustPrintPassTimings();
         }
     }
+    fn print_statistics(&self) {
+        unsafe {
+            llvm::LLVMRustPrintStatistics();
+        }
+    }
     fn run_link(
         cgcx: &CodegenContext<Self>,
         diag_handler: &Handler,

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1870,6 +1870,9 @@ extern "C" {
     /// Print the pass timings since static dtors aren't picking them up.
     pub fn LLVMRustPrintPassTimings();
 
+    /// Print the statistics since static dtors aren't picking them up.
+    pub fn LLVMRustPrintStatistics();
+
     pub fn LLVMStructCreateNamed(C: &Context, Name: *const c_char) -> &Type;
 
     pub fn LLVMStructSetBody<'a>(

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -110,6 +110,10 @@ unsafe fn configure_llvm(sess: &Session) {
         // Use non-zero `import-instr-limit` multiplier for cold callsites.
         add("-import-cold-multiplier=0.1", false);
 
+        if sess.print_llvm_stats() {
+            add("-stats", false);
+        }
+
         for arg in sess_args {
             add(&(*arg), true);
         }