AVR: Fix PR120423 / PR116389.

The problem with PR120423 and PR116389 is that reload might assign an invalid
hard register to a paradoxical subreg.  For example with the test case from
the PR, it assigns (REG:QI 31) to the inner of (subreg:HI (QI) 0) which is
valid, but the subreg will be turned into (REG:HI 31) which is invalid
and triggers an ICE in postreload.

The problem only occurs with the old reload pass.

The patch maps the paradoxical subregs to a zero-extends which will be
allocated correctly.  For the 120423 testcases, the code is the same like
with -mlra (which doesn't implement the fix), so the patch doesn't even
introduce a performance penalty.

The patch is only needed for v15:  v14 is not affected, and in v16 reload
will be removed.

	PR rtl-optimization/120423
	PR rtl-optimization/116389
gcc/
	* config/avr/avr.md [-mno-lra]: Add pre-reload split to transform
	(left shift of) a paradoxical subreg to a (left shift of) zero-extend.
gcc/testsuite/
	* gcc.target/avr/torture/pr120423-1.c: New test.
	* gcc.target/avr/torture/pr120423-2.c: New test.
	* gcc.target/avr/torture/pr120423-116389.c: New test.