all: add GFNI instructions (#344)

Adds support for the GFNI "Galois Field New Instructions" instruction set.

These instructions are not included in the Opcodes database, therefore they're
added using the "extras" mechanism introduced in #345.

For simplicity, the loading phase is updated slightly so that AVX-512 form
expansion rules are applied after extras are added to the list. This greatly
reduces the number of forms that have to be specified by hand.

Based on #343
Fixes #335

Co-authored-by: Klaus Post <klauspost@gmail.com>

internal/inst/types.go

@@ -40,6 +40,13 @@ func (i Instruction) IsConditionalBranch() bool {
 	return i.IsBranch() && i.Opcode != "JMP"
 }
 
+// Clone the instruction.
+func (i Instruction) Clone() Instruction {
+	c := i
+	c.Forms = i.Forms.Clone()
+	return c
+}
+
 // Forms is a collection of instruction forms.
 type Forms []Form
 
@@ -78,6 +85,15 @@ func (fs Forms) IsNiladic() bool {
 	return len(a) == 1 && a[0] == 0
 }
 
+// Clone the instruction forms.
+func (fs Forms) Clone() Forms {
+	cs := make(Forms, 0, len(fs))
+	for _, f := range fs {
+		cs = append(cs, f.Clone())
+	}
+	return cs
+}
+
 // Form specifies one accepted set of operands for an instruction.
 type Form struct {
 	// Instruction sets this instruction form requires.