cmd/compile: make sure field offset is aligned for structure splitting
cmd/compile: make sure field offset is aligned for structure splitting

Always use the aligned form -- the old code sometimes could
at least nominally use a misaligned field in an SSA-able struct,
even if not actually.

Fixes #42181.

Change-Id: Ibdce0985f9349da70921a37423054b85ee4200d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/265277
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Trust: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,nawawi/go