compiler: clean up pointer pack code

This means runtime.alloc isn't allocated every time but instead only
when it is actually needed. It also just cleans up the compiler code a
little bit, and makes the next commit a bit nicer.

compiler/llvm.go

@@ -129,11 +129,9 @@ func (b *builder) emitPointerPack(values []llvm.Value) llvm.Value {
 		// Packed data is bigger than a pointer, so allocate it on the heap.
 		sizeValue := llvm.ConstInt(b.uintptrType, size, false)
 		align := b.targetData.ABITypeAlignment(packedType)
-		alloc := b.mod.NamedFunction("runtime.alloc")
-		packedAlloc := b.CreateCall(alloc.GlobalValueType(), alloc, []llvm.Value{
+		packedAlloc := b.createRuntimeCall("alloc", []llvm.Value{
 			sizeValue,
 			llvm.ConstNull(b.dataPtrType),
-			llvm.Undef(b.dataPtrType), // unused context parameter
 		}, "")
 		packedAlloc.AddCallSiteAttribute(0, b.ctx.CreateEnumAttribute(llvm.AttributeKindID("align"), uint64(align)))
 		if b.NeedsStackObjects {