[RISCV] Remove RISCVISD::SPLAT_VECTOR_I64 in favor of RISCVISD::VMV_V_X_VL.

SPLAT_VECTOR_I64 has the same semantics as RISCVISD::VMV_V_X_VL, it
just assumed VLMax instead of carrying a VL operand.

Include order of RISCVInstrInfoVSDPatterns.td and RISCVInstrInfoVVLPatterns.td
has been swapped to avoid moving riscv_vmv_v_x_vl into
RISCVInstrInfoVSDPatterns.td and to allow moving other "_vl" SDNodes back to
RISCVInstrInfoVVLPatterns.td

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D118841