Fix (x::Vec)^n for n>3

[dubosipsl]

Fixes:

julia> SIMD.Vec(randn(Float64,4)...)^4
ERROR: MethodError: no method matching powi(::NTuple{4, VecElement{Float64}}, ::Int64)

Or is there a good reason to not define powi for Int64 exponents ?

[KristofferC]

Looking at https://llvm.org/docs/LangRef.html#llvm-powi-intrinsic it says:

Generally, the only supported type for the exponent is the one matching with the C type int.

but we could maybe do some conversion on the Julia side to bring it down to a 32 bit int before passing it to LLVM.

[dubosipsl]

I have pushed another, pure Julia approach (no LLVM intrinsics). Is that better ?