[RISCV] Use a ComplexPattern to merge isel patterns for vector load/store with...

[RISCV] Use a ComplexPattern to merge isel patterns for vector load/store with GPR and FrameIndex addresses.

This reduces the isel table size by about 3000 bytes.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D95844