rustc_trans: 'assume' intrinsic is only available on LLVM >= 3.6

Based on the patch from Luca Bruno.

Instead of creating an empty C function in the rt, this version creates an shim
noop function using llvm. This function is declared as internal, and the
unsupported assume intrinsic and the shim gets completly removed by the
optimizer.

Author: davidvoit

src/librustc_trans/trans/context.rs

@@ -933,11 +933,33 @@ fn declare_intrinsic(ccx: &CrateContext, key: & &'static str) -> Option<ValueRef
     ifn!("llvm.lifetime.end", fn(t_i64, i8p) -> void);
 
     ifn!("llvm.expect.i1", fn(i1, i1) -> i1);
-    ifn!("llvm.assume", fn(i1) -> void);
 
     // Some intrinsics were introduced in later versions of LLVM, but they have
     // fallbacks in libc or libm and such.
     macro_rules! compatible_ifn {
+        ($name:expr, noop($cname:ident ($($arg:expr),*) -> void), $llvm_version:expr) => (
+            if unsafe { llvm::LLVMVersionMinor() >= $llvm_version } {
+                // The `if key == $name` is already in ifn!
+                ifn!($name, fn($($arg),*) -> void);
+            } else if *key == $name {
+                let f = declare::declare_cfn(ccx, stringify!($cname),
+                                             Type::func(&[$($arg),*], &void),
+                                             ty::mk_nil(ccx.tcx()));
+                llvm::SetLinkage(f, llvm::InternalLinkage);
+
+                let bld = ccx.builder();
+                let llbb = unsafe {
+                    llvm::LLVMAppendBasicBlockInContext(ccx.llcx(), f,
+                                                        "entry-block\0".as_ptr() as *const _)
+                };
+
+                bld.position_at_end(llbb);
+                bld.ret_void();
+
+                ccx.intrinsics().borrow_mut().insert($name, f.clone());
+                return Some(f);
+            }
+        );
         ($name:expr, $cname:ident ($($arg:expr),*) -> $ret:expr, $llvm_version:expr) => (
             if unsafe { llvm::LLVMVersionMinor() >= $llvm_version } {
                 // The `if key == $name` is already in ifn!
@@ -952,6 +974,8 @@ fn declare_intrinsic(ccx: &CrateContext, key: & &'static str) -> Option<ValueRef
         )
     }
 
+    compatible_ifn!("llvm.assume", noop(llvmcompat_assume(i1) -> void), 6);
+
     if ccx.sess().opts.debuginfo != NoDebugInfo {
         ifn!("llvm.dbg.declare", fn(Type::metadata(ccx), Type::metadata(ccx)) -> void);
         ifn!("llvm.dbg.value", fn(Type::metadata(ccx), t_i64, Type::metadata(ccx)) -> void);