Inline get_reg_value method of dwarf_expr_context

Inline get_reg_value method of dwarf_expr_context

The get_reg_value method is a small function that is only called once,
so it can be inlined to simplify the dwarf_expr_context class.

gdb/ChangeLog:

	* dwarf2/expr.c (dwarf_expr_context::get_reg_value): Remove
	method.
	(dwarf_expr_context::execute_stack_op): Inline get_reg_value
	method.
	* dwarf2/expr.h (dwarf_expr_context::get_reg_value): Remove
	method.

Change-Id: I7bee8587d30983355d371f0769068f712cfbce1c