[Matrix] Add the matrix test from D83570. NFC.

Author: Sjoerd Meijer

clang/test/CodeGen/matrix-lowering-opt-levels.c

@@ -0,0 +1,20 @@
+// RUN: %clang -O0 -fenable-matrix -S -emit-llvm %s -o - | FileCheck  %s
+// RUN: %clang -O1 -fenable-matrix -S -emit-llvm %s -o - | FileCheck  %s
+// RUN: %clang -O2 -fenable-matrix -S -emit-llvm %s -o - | FileCheck  %s
+// RUN: %clang -O3 -fenable-matrix -S -emit-llvm %s -o - | FileCheck  %s
+// RUN: %clang -Ofast -fenable-matrix -S -emit-llvm %s -o - | FileCheck  %s
+// RUN: %clang -Os -fenable-matrix -S -emit-llvm %s -o - | FileCheck  %s
+// RUN: %clang -Oz -fenable-matrix -S -emit-llvm %s -o - | FileCheck  %s
+
+// Smoke test that the matrix intrinsics are lowered at any optimisation level.
+
+typedef float m4x4_t __attribute__((matrix_type(4, 4)));
+
+m4x4_t f(m4x4_t a, m4x4_t b, m4x4_t c) {
+  //
+  // CHECK-LABEL: f(
+  // CHECK-NOT:     @llvm.matrix
+  // CHECK:       }
+  //
+  return a + b * c;
+}