[PR40457] [arm] expand SI-aligned movdi into pair of movsi

[PR40457] [arm] expand SI-aligned movdi into pair of movsi

When expanding a misaligned DImode move, emit aligned SImode moves if
the parts are sufficiently aligned.  This enables neighboring stores
to be peephole-combined into stm, as expected by the PR40457 testcase,
even after SLP vectorizes the originally aligned SImode stores into a
misaligned DImode store.

for  gcc/ChangeLog

	PR target/40457
	* config/arm/arm.md (movmisaligndi): Prefer aligned SImode
	moves.