[PowerPC] Exploit `vrl(b|h|w|d)` to perform vector rotation

Summary:
Currently, we set legalization action of `ISD::ROTL` vectors as
`Expand` in `PPCISelLowering`. However, we can exploit `vrl(b|h|w|d)`
to lower `ISD::ROTL` directly.

Differential Revision: https://reviews.llvm.org/D71324