This adds methods SelectFromPair for {Int,Uint,Float}32x4
and SelectFromPairGrouped for {Int,Uint,Float}32x8.
Each of these has the signature
```
func(x T32xK.Method(a,b,c,d uint8, y T32xK) T32xK)
```
where a, b, c, d can be 0-7 and each one specifies an
element from the concatenated elements of x (0-3) and
y (4-7). When a, b, c, d are constants, 1 or 2
instructions are generated, otherwise, it's done the
harder-slower way with a function call.