Add an operation that checks whether we are at the start of the string.

src/bytecodes-irregexp.h

Index: src/bytecodes-irregexp.h
===================================================================
--- src/bytecodes-irregexp.h	(revision 864)
+++ src/bytecodes-irregexp.h	(working copy)
@@ -64,7 +64,8 @@
 V(LOOKUP_MAP8,       29, 99) /* l_map8 start16 byte_map addr32*             */ \
 V(LOOKUP_HI_MAP8,    30, 99) /* l_himap8 start8 byte_map_addr32 addr32*     */ \
 V(CHECK_REGISTER_LT, 31, 8) /* check_reg_lt register_index value16 addr32   */ \
-V(CHECK_REGISTER_GE, 32, 8) /* check_reg_ge register_index value16 addr32   */
+V(CHECK_REGISTER_GE, 32, 8) /* check_reg_ge register_index value16 addr32   */ \
+V(CHECK_NOT_AT_START, 33, 5) /* check_not_at_start addr32                   */
 
 #define DECLARE_BYTECODES(name, code, length) \
   static const int BC_##name = code;