[RISCV] Improve support for forming widening multiplies when one input is a scalar splat.

If one input of a fixed vector multiply is a sign/zero extend and
the other operand is a splat of a scalar, we can use a widening
multiply if the scalar value has sufficient sign/zero bits.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D110028