[ARM] Add missing pseudo-instruction for Thumb1 RSBS.

Shows up rarely for 64-bit arithmetic, more frequently for the compare
patterns added in r325323.

Differential Revision: https://reviews.llvm.org/D53848

llvm-svn: 345782
diff --git a/llvm/lib/Target/ARM/ARMInstrThumb.td b/llvm/lib/Target/ARM/ARMInstrThumb.td
index 8b85db7..3c15362 100644
--- a/llvm/lib/Target/ARM/ARMInstrThumb.td
+++ b/llvm/lib/Target/ARM/ARMInstrThumb.td
@@ -1343,6 +1343,12 @@
                                                            tGPR:$Rm))]>,
                 Requires<[IsThumb1Only]>,
                 Sched<[WriteALU]>;
+
+  def tRSBS   : tPseudoInst<(outs tGPR:$Rd), (ins tGPR:$Rn),
+                            2, IIC_iALUr,
+                            [(set tGPR:$Rd, CPSR, (ARMsubc 0, tGPR:$Rn))]>,
+                Requires<[IsThumb1Only]>,
+                Sched<[WriteALU]>;
 }
 
 // Sign-extend byte