rustc_trans: don't hardcode llvm version for conditional intrinsics

This commit introduce a third parameter for compatible_ifn!, as new
intrinsics are being added in recent LLVM releases and there is no
need to hardcode a specific case.

Signed-off-by: Luca Bruno <lucab@debian.org>

src/librustc_llvm/lib.rs

@@ -1772,6 +1772,8 @@ extern {
                          -> ValueRef;
 
     pub fn LLVMRustDebugMetadataVersion() -> u32;
+    pub fn LLVMVersionMajor() -> u32;
+    pub fn LLVMVersionMinor() -> u32;
 
     pub fn LLVMRustAddModuleFlag(M: ModuleRef,
                                  name: *const c_char,