[RISCV] Cap legal fixed-length vectors to 256-element types

Previously, RISC-V would make legal all fixed-length vectors types whose
size are less than or equal to some function of the minimum value of
VLEN and the maximum-permissible LMUL grouping.

Due to vector legalization issues, this patch instead caps the legal
fixed-length vector types to those with 256 elements. This value was
chosen because it is the longest vector length which has corresponding
MVTs across all supported element types.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D101839