[SLP] Add cost model for `llvm.powi.*` intrinsics (REAPPLIED)

Patch was reverted in 4c5f10ae due to buildbot failures, now being
reapplied with updated AArch64 and RISCV tests.

This patch adds handling for the llvm.powi.* intrinsics in
BasicTTIImplBase::getIntrinsicInstrCost() and improves vectorization.
Closes #53887.

Differential Revision: https://reviews.llvm.org/D128172