[PowerPC][Power10] Exploit the xxsplti32dx instruction when lowering VECTOR_SHUFFLE.

[PowerPC][Power10] Exploit the xxsplti32dx instruction when lowering VECTOR_SHUFFLE.

This patch aims to exploit the xxsplti32dx XT, IX, IMM32 instruction when lowering VECTOR_SHUFFLEs.
We implement lowerToXXSPLTI32DX when lowering vector shuffles to check if:
- Element size is 4 bytes
- The RHS is a constant vector (and constant splat of 4-bytes)
- The shuffle mask is a suitable mask for the XXSPLTI32DX instruction where it is one of the 32 masks:
<0, 4-7, 2, 4-7>
<4-7, 1, 4-7, 3>

Differential Revision: https://reviews.llvm.org/D83245