[X86] Prevent clang clobber checking for asm flag constraints.

Update getConstraintRegister as X86 Asm flag output constraints are no
longer fully alphanumeric,

llvm-svn: 354211

clang/lib/Basic/Targets/X86.h

@@ -198,7 +198,7 @@ public:
                                   StringRef Expression) const override {
     StringRef::iterator I, E;
     for (I = Constraint.begin(), E = Constraint.end(); I != E; ++I) {
-      if (isalpha(*I))
+      if (isalpha(*I) || *I == '@')
         break;
     }
     if (I == E)

clang/test/CodeGen/inline-asm-x86-flag-output.c

@@ -363,3 +363,14 @@ int test_ccs(long nr, volatile long *addr) {
     return 0;
   return 1;
 }
+
+_Bool check_no_clobber_conflicts() {
+  //CHECK-LABEL: @check_no_clobber_conflicts
+  //CHECK:  = tail call i8 asm "", "={@cce},~{cx},~{dirflag},~{fpsr},~{flags}"()
+  _Bool b;
+  asm(""
+      : "=@cce"(b)
+      :
+      : "cx");
+  return b;
+}