all: AVX-512 (#217)

Extends avo to support most AVX-512 instruction sets.

The instruction type is extended to support suffixes. The K family of opmask
registers is added to the register package, and the operand package is updated
to support the new operand types. Move instruction deduction in `Load` and
`Store` is extended to support KMOV* and VMOV* forms.

Internal code generation packages were overhauled. Instruction database loading
required various messy changes to account for the additional complexities of the
AVX-512 instruction sets. The internal/api package was added to introduce a
separation between instruction forms in the database, and the functions avo
provides to create them. This was required since with instruction suffixes there
is no longer a one-to-one mapping between instruction constructors and opcodes.

AVX-512 bloated generated source code size substantially, initially increasing
compilation and CI test times to an unacceptable level. Two changes were made to
address this:

1.  Instruction constructors in the `x86` package moved to an optab-based
    approach. This compiles substantially faster than the verbose code
    generation we had before.

2.  The most verbose code-generated tests are moved under build tags and
    limited to a stress test mode. Stress test builds are run on
    schedule but not in regular CI.

An example of AVX-512 accelerated 16-lane MD5 is provided to demonstrate and
test the new functionality.

Updates #20 #163 #229

Co-authored-by: Vaughn Iverson <vsivsi@yahoo.com>

build/context.go

@@ -15,6 +15,9 @@ import (
 	"github.com/mmcloughlin/avo/reg"
 )
 
+//go:generate avogen -output zinstructions.go build
+//go:generate avogen -output zinstructions_test.go buildtest
+
 // Context maintains state for incrementally building an avo File.
 type Context struct {
 	pkg      *packages.Package
@@ -175,8 +178,6 @@ func (c *Context) activefunc() *ir.Function {
 	return c.function
 }
 
-//go:generate avogen -output zinstructions.go build
-
 // StaticGlobal adds a new static data section to the file and returns a pointer to it.
 func (c *Context) StaticGlobal(name string) operand.Mem {
 	c.global = ir.NewStaticGlobal(name)