[AMDGPU] Add test for instructions unsupported on gfx940. NFC.

Author: rampitec

llvm/test/MC/AMDGPU/gfx940_err.s

@@ -0,0 +1,17 @@
+// RUN: not llvm-mc -arch=amdgcn -mcpu=gfx940 %s 2>&1 | FileCheck --check-prefix=GFX940 --implicit-check-not=error: %s
+
+v_mac_f32 v0, v1, v2
+// FIXME: error message is incorrect
+// GFX940: error: operands are not valid for this GPU or mode
+
+v_mad_f32 v0, v1, v2, v3
+// GFX940: error: instruction not supported on this GPU
+
+v_madak_f32 v0, v1, v2, 0
+// GFX940: error: instruction not supported on this GPU
+
+v_madmk_f32 v0, v1, 0, v2
+// GFX940: error: instruction not supported on this GPU
+
+v_mad_legacy_f32 v0, v1, v2, v3
+// GFX940: error: instruction not supported on this GPU