Re-apply [1346], which was reverted in [1348], now that padding should no longer

be a problem.
As noted before, this will break ABI-compatibility on x86-64, so recompile old
code before linking with newly-compiled code if you're on x86-64.
As an added bonus, I actually tested it this time :).

gen/abi-x86-64.cpp

@@ -44,7 +44,6 @@
 #include "gen/abi.h"
 #include "gen/abi-x86-64.h"
 #include "gen/structs.h"
-//#include "gen/llvm-version.h"         // only use is commented out.
 #include "ir/irfunction.h"
 
 #include <cassert>
@@ -402,19 +401,6 @@ 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);
     }