cmd/asm: encode instructions like SHA1SU0 with a separate case for arm64

cmd/asm: encode instructions like SHA1SU0 with a separate case for arm64

Before this CL, instructions such as SHA1SU0, AESD and AESE are encoded
in case 1 together with FMOV/ADD, and some error checking is missing,
for example:

  SHA1SU0       V1.B16, V2.B16, V3.B16   // wrong data arrangement
  SHA1SU0       V1.4S, V2.S4, V3.S4      // correct

Both will be accepted by the assembler, but the first one is totally
incorrect.

This CL fixes these potential encoding issues by moving them into
separate cases, adds some error tests, and also fixes a wrong encoding
operand for ASHA1C.

Change-Id: Ic778321a567735d48bc34a1247ee005c4ed9e11f
Reviewed-on: https://go-review.googlesource.com/c/go/+/493195
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>