[Patch,AVR]: Better support CPSE (a bit)

[Georg-Johann Lay]

This patch aims at better support of CPSE instruction in cases where otherwise
code like

     TST   Rn     ; *cmpqi
     BREQ  .+2    ; branch
     RJMP  .Lm

would be produced.  As branch is not a single instruction, it cannot be skipped.

peephole seems to be the only feature that can do this optimization. I tried a
combine pattern and it works will and is straight forward, but then it's hard
to detect the situations where TST is superfluous because some arithmetic
instruct already set cc0.

I also tried movqicc to emit code and to prefer jumps that please CPSE, but the
middle end has it's own understanding of what the code should look like and
it's merely impossible to work against that in the backend. Moreover there was
changes (like for PR45416 to feature specific target) that turned out to give
worse code for AVR. But as so often the "what the hell is AVR?" target is left
behind.

So here is the patch even though I really don't like peephole[2]. It passes
without regressions and has a bit of code cleanup, too.

Ok to apply?

Johann

	* config/avr/avr-protos.h (lpm_reg_rtx, lpm_addr_reg_rtx,
	tmp_reg_rtx, zero_reg_rtx, all_regs_rtx, rampz_rtx): Make global.
	* config/avr/avr.c: Ditto.
	(avr_regnames): Remove because unused.
	* config/avr/avr.md (*cpse.ne): New peephole.
	(*cpse.eq): New peephole from former cpse peepholes.
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