librustc_trans: Gate the preinliner with another -Z flag.

Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

src/librustc/session/config.rs

@@ -1254,6 +1254,9 @@ fn parse_edition(slot: &mut Edition, v: Option<&str>) -> bool {
          location if it's empty."),
     pgo_use: String = (String::new(), parse_string, [TRACKED],
         "Use PGO profile data from the given profile file."),
+    disable_instrumentation_preinliner: bool =
+        (false, parse_bool, [TRACKED], "Disable the instrumentation pre-inliner, \
+        useful for profiling / PGO."),
     relro_level: Option<RelroLevel> = (None, parse_relro_level, [TRACKED],
         "choose which RELRO level to use"),
     nll: bool = (false, parse_bool, [UNTRACKED],

src/librustc_trans/llvm_util.rs

@@ -61,7 +61,9 @@ unsafe fn configure_llvm(sess: &Session) {
         add("rustc"); // fake program name
         if sess.time_llvm_passes() { add("-time-passes"); }
         if sess.print_llvm_passes() { add("-debug-pass=Structure"); }
-        add("-disable-preinline");
+        if sess.opts.debugging_opts.disable_instrumentation_preinliner {
+            add("-disable-preinline");
+        }
 
         for arg in &sess.opts.cg.llvm_args {
             add(&(*arg));