Allow DW_OP_GNU_uninit in dwarf_expr_require_composition

Allow DW_OP_GNU_uninit in dwarf_expr_require_composition

In DWARF expression handling, some operators are required to be either
at the end of an expression or followed by a composition operator.  So
far only the operators DW_OP_reg0-31 were allowed to be followed by
DW_OP_GNU_uninit instead, and particularly DW_OP_regx was not, which is
obviously inconsistent.

This patch allows DW_OP_GNU_uninit after all operators requiring a
composition, to simplify the code and make it more consistent.  This
policy may be more permissive than necessary, but in the worst case just
leads to a DWARF location description resulting in an uninitialized
value instead of an error message.

gdb/ChangeLog:

	* dwarf2expr.c (dwarf_expr_require_composition): Allow
	DW_OP_GNU_uninit.
	(execute_stack_op): Use dwarf_expr_require_composition instead of
	copying its logic.