[CodeGen][SVE] Legalisation of extends with scalable types

Summary:
This patch adds legalisation of extensions where the operand
of the extend is a legal scalable type but the result is not.

EXTRACT_SUBVECTOR is used to split the result, before
being replaced by target-specific [S|U]UNPK[HI|LO] operations.

For example:

```
zext <vscale x 16 x i8> %a to <vscale x 16 x i16>
```
should emit:

```
uunpklo z2.h, z0.b
uunpkhi z1.h, z0.b
```

Reviewers: sdesmalen, efriedma, david-arm

Reviewed By: efriedma

Subscribers: tschuett, hiraditya, rkruppe, psnobl, huihuiz, cfe-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79587