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_ssa/src/back/write.rs

@@ -1945,6 +1945,10 @@ impl<B: ExtraBackendMethods> OngoingCodegen<B> {
             self.backend.print_pass_timings()
         }
 
+        if sess.print_llvm_stats() {
+            self.backend.print_statistics()
+        }
+
         (
             CodegenResults {
                 metadata: self.metadata,

compiler/rustc_codegen_ssa/src/traits/write.rs

@@ -35,6 +35,7 @@ pub trait WriteBackendMethods: 'static + Sized + Clone {
         cached_modules: Vec<(SerializedModule<Self::ModuleBuffer>, WorkProduct)>,
     ) -> Result<(Vec<LtoModuleCodegen<Self>>, Vec<WorkProduct>), FatalError>;
     fn print_pass_timings(&self);
+    fn print_statistics(&self);
     unsafe fn optimize(
         cgcx: &CodegenContext<Self>,
         diag_handler: &Handler,