[AMDGPU] Fix regression with vectorization limiting

D67148 has removed TTI::getNumberOfRegisters(bool Vector) and
started to call TTI::getNumberOfRegisters(unsigned ClassID) from
the LoopVectorize. This has resulted in an unrestricted vectorization
on AMDGPU blowing up register pressure.

Differential Revision: https://reviews.llvm.org/D122850