[ValueTypes] Add MVTs for v256i16 and v256f16

This patch adds the two MVTs to fix a legalizer crash when using vector
shuffles of <256 x i16> and <128 x i16> on RISC-V. The legalizer can't
promote the operand of `v256i32 = any_extend_vector_inreg v128i16`.

Reviewed By: craig.topper, RKSimon

Differential Revision: https://reviews.llvm.org/D101769