This change makes it easier to express instructions
with arbitrary number of operands.

Rationale: previous approach with operand "hiding" does
not scale well, AVX and especially AVX512 have many
instructions with 3+ operands.

x86 asm backend is updated to handle up to 6 explicit operands.
It also fixes issue with 4-th immediate operand type checks.
All `ytab` tables are updated accordingly.

Changes to non-x86 backends only include these patterns:
`p.From3 = X` => `p.SetFrom3(X)`
`p.From3.X = Y` => `p.GetFrom3().X = Y`

Over time, other backends can adapt Prog.RestArgs
and reduce the amount of workarounds.

-- Performance --

x/benchmark/build:

$ benchstat upstream.bench patched.bench
name      old time/op                 new time/op                 delta
Build-48                  21.7s ± 2%                  21.8s ± 2%   ~     (p=0.218 n=10+10)

name      old binary-size             new binary-size             delta
Build-48                  10.3M ± 0%                  10.3M ± 0%   ~     (all equal)

name      old build-time/op           new build-time/op           delta
Build-48                  21.7s ± 2%                  21.8s ± 2%   ~     (p=0.218 n=10+10)

name      old build-peak-RSS-bytes    new build-peak-RSS-bytes    delta
Build-48                  145MB ± 5%                  148MB ± 5%   ~     (p=0.218 n=10+10)

name      old build-user+sys-time/op  new build-user+sys-time/op  delta
Build-48                  21.0s ± 2%                  21.2s ± 2%   ~     (p=0.075 n=10+10)

Microbenchmark shows a slight slowdown.

name        old time/op  new time/op  delta
AMD64asm-4  49.5ms ± 1%  49.9ms ± 1%  +0.67%  (p=0.001 n=23+15)

func BenchmarkAMD64asm(b *testing.B) {
  for i := 0; i < b.N; i++ {
    TestAMD64EndToEnd(nil)
    TestAMD64Encoder(nil)
  }
}

Change-Id: I4f1d37b5c2c966da3f2127705ccac9bff0038183
Reviewed-on: https://go-review.googlesource.com/63490
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>