Compiler/assembler fix

There are three Thumb encodings for push and pop, and one of
them (Thumb2/t2) requires at least two registers in the push/pop
list.  This CL adds the single-register encoding (which is really
just another name for a standard base-modifying load/store).

Change-Id: I963c472a0d5f288654417dbd85758d6b5cc61388
diff --git a/src/compiler/codegen/arm/ArmLIR.h b/src/compiler/codegen/arm/ArmLIR.h
index daa1396..1e4022e 100644
--- a/src/compiler/codegen/arm/ArmLIR.h
+++ b/src/compiler/codegen/arm/ArmLIR.h
@@ -693,6 +693,8 @@
     kThumb2SubsRRI12,    /* setflags encoding */
     kThumb2OrrRRRs,      /* orrx [111010100101] rn[19..16] [0000] rd[11..8]
                                    [0000] rm[3..0] */
+    kThumb2Push1,        /* t3 encoding of push */
+    kThumb2Pop1,         /* t3 encoding of pop */
     kArmLast,
 } ArmOpcode;