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 <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: David Chase <drchase@google.com>