* config/tc-arm.c (create_register_alias): Return a boolean rather than an integer.

* config/tc-arm.c (create_register_alias): Return a boolean rather than an integer.
  Check the return value of insert_reg_alias and do not continue to create aliases once an insertion has failed.
  (s_unreq): Delete the all-upper-case and all-lower-case alternatives as well.
* testsuite/gas/arm/arm.s: Add tests for re-aliasing a previously removed alias.
* testsuite/gas/arm/arm.l: Add new expected warning message.