ARM Assembler support for DMB instruction.

Flesh out the options supported for the instruction. Shuffle tests a bit and
add entries for the rest of the options. Add an alias to handle the default
operand of "sy".

llvm-svn: 135109
diff --git a/llvm/lib/Target/ARM/ARMInstrInfo.td b/llvm/lib/Target/ARM/ARMInstrInfo.td
index 29ab4ea..c4afe40 100644
--- a/llvm/lib/Target/ARM/ARMInstrInfo.td
+++ b/llvm/lib/Target/ARM/ARMInstrInfo.td
@@ -3304,6 +3304,8 @@
 }
 }
 
+def : InstAlias<"dmb", (DMB 0xf)>;
+
 def DSB : AInoP<(outs), (ins memb_opt:$opt), MiscFrm, NoItinerary,
                 "dsb", "\t$opt", []>,
                 Requires<[IsARM, HasDB]> {