cmd/compile: don't ICE on unaligned offsets for pointer writes

cmd/compile: don't ICE on unaligned offsets for pointer writes

User code is unlikely to be correct, but don't crash the compiler
when the offset of a pointer in an object is not a multiple of the
pointer size.

Fixes #61187

Change-Id: Ie56bfcb38556c5dd6f702ae4ec1d4534c6acd420
Reviewed-on: https://go-review.googlesource.com/c/go/+/508555
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>