R600: Add missing Southern Islands GPU to setCPU() function

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178498 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Basic/Targets.cpp

@@ -1492,7 +1492,7 @@ public:
       .Case("caicos",   GK_NORTHERN_ISLANDS)
       .Case("cayman",   GK_CAYMAN)
       .Case("aruba",    GK_CAYMAN)
-      .Case("SI",       GK_SOUTHERN_ISLANDS)
+      .Case("tahiti",   GK_SOUTHERN_ISLANDS)
       .Case("pitcairn", GK_SOUTHERN_ISLANDS)
       .Case("verde",    GK_SOUTHERN_ISLANDS)
       .Case("oland",    GK_SOUTHERN_ISLANDS)