CostModel/RISCV: tweak cost of vector ctpop under ZVBB (#67020)

Under RISCV experimental-zvbb, vector variants of llvm.ctpop lower to a
single instruction: vcpop. The cost-model does not check for the ZVBB
extension, and always associates a high cost to vector variants of
llvm.ctpop. Fix this defect.

GitOrigin-RevId: 7c128f6d0e358ccf4332426b829e8f2eef9adacb
2 files changed