[ArgPromotion] Add test for volatile and atomic loads (NFC)

Argument promotion does handle these correctly (by not promoting
them), but there were no tests to ensure this.

Author: nikic

llvm/test/Transforms/ArgumentPromotion/volatile-atomic.ll

@@ -0,0 +1,40 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -argpromotion < %s | FileCheck %s
+
+; Make sure volatile and atomic loads are not promoted.
+
+define internal i32 @callee_volatile(i32* %p) {
+; CHECK-LABEL: @callee_volatile(
+; CHECK-NEXT:    [[V:%.*]] = load volatile i32, i32* [[P:%.*]], align 4
+; CHECK-NEXT:    ret i32 [[V]]
+;
+  %v = load volatile i32, i32* %p
+  ret i32 %v
+}
+
+define void @caller_volatile(i32* %p) {
+; CHECK-LABEL: @caller_volatile(
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @callee_volatile(i32* [[P:%.*]])
+; CHECK-NEXT:    ret void
+;
+  call i32 @callee_volatile(i32* %p)
+  ret void
+}
+
+define internal i32 @callee_atomic(i32* %p) {
+; CHECK-LABEL: @callee_atomic(
+; CHECK-NEXT:    [[V:%.*]] = load atomic i32, i32* [[P:%.*]] seq_cst, align 4
+; CHECK-NEXT:    ret i32 [[V]]
+;
+  %v = load atomic i32, i32* %p seq_cst, align 4
+  ret i32 %v
+}
+
+define void @caller_atomic(i32* %p) {
+; CHECK-LABEL: @caller_atomic(
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @callee_atomic(i32* [[P:%.*]])
+; CHECK-NEXT:    ret void
+;
+  call i32 @callee_atomic(i32* %p)
+  ret void
+}