[RISCV] Check floating point vector instruction with SEW=64 is valid when vsetvl insertion

Scalar move and splat instruction are only demand the SEW is greater than
its own needs, but floating point vector with SEW=64 is not alwaws valid even
SEW=64 is valid, because we have a special configuration: zve64f.

So we need to check floating point vector instruction with SEW=64 is
valid when compute demand of floating point scalar move and splat
instruction.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D158086