expansion: Sign or zero extend on MEM_REF stores into SUBREG with SUBREG_PROMOTED_VAR...

expansion: Sign or zero extend on MEM_REF stores into SUBREG with SUBREG_PROMOTED_VAR_P [PR98190]

Some targets decide to promote certain scalar variables to wider mode,
so their DECL_RTL is a SUBREG with SUBREG_PROMOTED_VAR_P.
When storing to such vars, store_expr takes care of sign or zero extending,
but if we store e.g. through MEM_REF into them, no sign or zero extension
happens and that leads to wrong-code e.g. on the following testcase on
aarch64-linux.

The following patch uses store_expr if we overwrite all the bits and it is
not reversed storage order, i.e. something that store_expr handles normally,
and otherwise (if the most significant bit is (or for pdp11 might be, but
pdp11 doesn't promote) being modified), the code extends manually.

2020-12-11  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/98190
	* expr.c (expand_assignment): If to_rtx is a promoted SUBREG,
	ensure sign or zero extension either through use of store_expr
	or by extending manually.

	* gcc.dg/pr98190.c: New test.