cmd/compile: adjust interface conversion function selection with 0-sized fields

0-sized fields do not affect how arguments are passed under the
register ABI. But it does affect the size and alignment of the
type, and may affect the choice of interface conversion function.
Specifically, struct { a [0]int32; b [4]byte } should be passed in
memory, therefore should not use convT32.

Change-Id: Idfa21af79b81c196b50253b0be1fa4edecd12b45
Reviewed-on: https://go-review.googlesource.com/c/go/+/308651


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>