Remove an old workaround that was kept around for ABI-compatibility on x86-64

between different LLVM versions.
This means LLVM r67588 is required if you want to compile for x86-64, otherwise
the backend will assert when you try to return e.g. struct { int i; char c; }
from a function. (In particular, this is no longer compatible with LLVM 2.5)
It also means that any code returning small structs on x86-64 will probably
need to be recompiled in order to be linkable to code compiled with this change.

gen/abi-x86-64.cpp

@@ -43,7 +43,6 @@
 #include "gen/llvmhelpers.h"
 #include "gen/abi.h"
 #include "gen/abi-x86-64.h"
-//#include "gen/llvm-version.h"         // only use is commented out.
 #include "ir/irfunction.h"
 
 #include <cassert>
@@ -401,19 +400,6 @@ namespace x86_64_D_cc {
     bool retStructInRegs(TypeStruct* st) {
         // 'fastcc' allows returns in up to two registers of each kind:
         DRegCount state(2, 2, 2);
-    #if 1 //LLVM_REV < 67588
-        // (If uncommenting the LLVM_REV line above, also uncomment llvm-version #include
-        
-        // LLVM before trunk r67588 doesn't allow a second int to be an i1 or
-        // i8. (See <http://llvm.org/PR3861>)
-        // Rather than complicating shouldPassStructInRegs(), just disallow
-        // second integers for now.
-        // FIXME: Disabling this for older LLVM only makes the abi dependent on
-        //        LLVM revision, which seems like a bad idea. We could extend
-        //        i8 parts to i16 to work around this issue until 2.6...
-        // TODO: Remove this workaround when support for LLVM 2.5 is dropped.
-        state.ints = 1;
-    #endif
         return shouldPassStructInRegs(st, state);
     }