[RISCV] Support vector types in combination with fastcc

This patch extends the RISC-V lowering of the 'fastcc' calling
convention to vector types, both fixed-length and scalable. Without this
patch, any function passing or returning vector types by value would
throw a compiler error.

Vectors are handled in 'fastcc' much as they are in the default calling
convention, the noticeable difference being the extended set of scalar
GPR registers that can be used to pass vectors indirectly.

Reviewed By: HsiangKai

Differential Revision: https://reviews.llvm.org/D102505