xtensa: Optimize stack frame adjustment more

xtensa: Optimize stack frame adjustment more

This patch introduces a convenient helper function for integer immediate
addition with scratch register as needed, that splits and emits either
up to two ADDI/ADDMI machine instructions or an addition by register
following an integer immediate load (which may later be transformed by
constantsynth).

By using the helper function, it makes stack frame adjustment logic
simplified and instruction count less in some cases.

gcc/ChangeLog:

	* config/xtensa/xtensa.cc
	(xtensa_split_imm_two_addends, xtensa_emit_add_imm):
	New helper functions.
	(xtensa_set_return_address, xtensa_output_mi_thunk):
	Change to use the helper function.
	(xtensa_emit_adjust_stack_ptr): Ditto.
	And also change to try reusing the content of scratch register
	A9 if the register is not modified in the function body.