When one passes short-lived fake rtxes to backends in order to test
their capabilities, it might be beneficial to allocate these rtxes on
stack in order to reduce the load on GC.

Provide macro counterparts of some of the gen_* functions for that
purpose.

gcc/ChangeLog:

2019-09-30  Ilya Leoshkevich  <iii@linux.ibm.com>

	* emit-rtl.c (init_raw_REG): New function.
	(gen_raw_REG): Use init_raw_REG.
	* gengenrtl.c (gendef): Emit init_* functions and alloca_*
	macros.
	* rtl.c (rtx_alloc_stat_v): Use rtx_init.
	* rtl.h (rtx_init): New function.
	(rtx_alloca): New function.
	(init_raw_REG): New function.
	(alloca_raw_REG): New macro.

From-SVN: r276303