[llvm-exegesis] Add mechanism to add target-specific passes.

Summary:
createX86FloatingPointStackifierPass is disabled until we handle
TracksLiveness correctly.

Reviewers: gchatelet

Subscribers: mgorny, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D48360

llvm-svn: 335117
diff --git a/llvm/tools/llvm-exegesis/lib/Target.cpp b/llvm/tools/llvm-exegesis/lib/Target.cpp
index 1b11f93..a69ab0c 100644
--- a/llvm/tools/llvm-exegesis/lib/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Target.cpp
@@ -14,10 +14,10 @@
 
 static ExegesisTarget* FirstTarget = nullptr;
 
-const ExegesisTarget* ExegesisTarget::lookup(llvm::StringRef TT) {
-  const llvm::Triple::ArchType Arch = llvm::Triple(TT).getArch();
+const ExegesisTarget *ExegesisTarget::lookup(llvm::Triple TT) {
   for (const ExegesisTarget* T = FirstTarget; T != nullptr; T = T->Next) {
-    if (T->matchesArch(Arch)) return T;
+    if (T->matchesArch(TT.getArch()))
+      return T;
   }
   return nullptr;
 }