[RISCV] Lower shuffles as rotates without zvbb

Now that the codegen for the expanded ISD::ROTL sequence has been improved,
it's probably profitable to lower a shuffle that's a rotate to the
vsll+vsrl+vor sequence to avoid a vrgather where possible, even if we don't
have the vror instruction.

This patch relaxes the restriction on ISD::ROTL being legal in
lowerVECTOR_SHUFFLEAsRotate. It also attempts to do the lowering twice: Once
if zvbb is enabled before any of the interleave/deinterleave/vmerge lowerings,
and a second time unconditionally just before it falls back to the vrgather.
This way it doesn't interfere with any of the above patterns that may be more
profitable than the expanded ISD::ROTL sequence.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D159353