Current assmbler accepts MUL* related instructions with 4 operands,
such as instruction "MUL R1, R2, R3, R4", which is illegal.

The fix adds an actual field informantion to Optab, which has value
of C_NONE, C_REG, etc, so assembler can use p.From3Type for checking
in oplook.

Add test cases.

Fixes #25059

Change-Id: I0656319383c460696b392197bf5960b987f8fc97
Reviewed-on: https://go-review.googlesource.com/109295Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>