[AVR] RTL prologue/epilogue

[Björn Haase]

Hello Andy,

I have tested your patch concerning RTL prologue/epilogue. Gratulations: My 
testsuite run only reports a single regression

Tests that now fail, but worked before:

gcc.c-torture/execute/20010122-1.c execution,  -O0 

. This happens on a testcase that anyway is problematic (succeeds only for 
selected set of optimization switches).

Due to your "rjmp" trick the resulting code also is sligtly tighter. IIUC, now 
there is also hope for optimizing sequences like

	call other_function
	ret

to 
        jmp other_function
.?

Yours,

Björn