Fix PR 71873 - ICE in push_reload

Fix PR 71873 - ICE in push_reload

Extend computation of subreg_in_class to constants and plus expressions
inside SUBREGs, before recursively calling push_reload. SYMBOL_REFs are
also CONSTANT_P, so remove explicit handling of SYMBOL_REFs.

gcc/ChangeLog

	PR target/71873
	* reload.c (push_reload): Compute subreg_in_class for
	subregs of constants and plus expressions. Remove special
	handling of SYMBOL_REFs.

gcc/testsuite/ChangeLog

	PR target/71873
	* gcc.target/avr/pr71873.c: New test.

From-SVN: r239321