AVR: PR117726 - Tweak 32-bit logical shifts of 25...30 for -Oz.

As it turns out, logical 32-bit shifts with an offset of 25..30 can
be performed in 7 instructions or less.  This beats the 7 instruc-
tions required for the default code of a shift loop.
Plus, with zero overhead, these cases can be 3-operand.

This is only relevant for -Oz because with -Os, 3op shifts are
split with -msplit-bit-shift (which is not performed with -Oz).

	PR target/117726
gcc/
	* config/avr/avr.cc (avr_ld_regno_p): New function.
	(ashlsi3_out) [case 25,26,27,28,29,30]: Handle and tweak.
	(lshrsi3_out): Same.
	(avr_rtx_costs_1) [SImode, ASHIFT, LSHIFTRT]: Adjust costs.
	* config/avr/avr.md (ashlsi3, *ashlsi3, *ashlsi3_const):
	Add "r,r,C4L" alternative.
	(lshrsi3, *lshrsi3, *lshrsi3_const): Add "r,r,C4R" alternative.
	* config/avr/constraints.md (C4R, C4L): New,
gcc/testsuite/
	* gcc.target/avr/torture/avr-torture.exp (AVR_TORTURE_OPTIONS):
	Turn one option variant into -Oz.