[AArch64] Remove check for Darwin that was needed to decide if x18 should
be reserved.

The decision to reserve x18 is going to be made solely by the front-end,
so it isn't necessary to check if the OS is Darwin in the backend.

llvm-svn: 243308
diff --git a/llvm/test/CodeGen/AArch64/arm64-stackmap.ll b/llvm/test/CodeGen/AArch64/arm64-stackmap.ll
index 1a4df7a..3eb1d27 100644
--- a/llvm/test/CodeGen/AArch64/arm64-stackmap.ll
+++ b/llvm/test/CodeGen/AArch64/arm64-stackmap.ll
@@ -1,5 +1,5 @@
-; RUN: llc -mtriple=arm64-apple-darwin                             < %s | FileCheck %s
-; RUN: llc -mtriple=arm64-apple-darwin -fast-isel -fast-isel-abort=1 < %s | FileCheck %s
+; RUN: llc -mtriple=arm64-apple-darwin -mattr=+reserve-x18                             < %s | FileCheck %s
+; RUN: llc -mtriple=arm64-apple-darwin -mattr=+reserve-x18 -fast-isel -fast-isel-abort=1 < %s | FileCheck %s
 ;
 ; Note: Print verbose stackmaps using -debug-only=stackmaps.