[RISCV] Look through copies when trying to find an implicit def in addVSetVL.

The InstrEmitter can sometimes insert a copy after an IMPLICIT_DEF
before connecting it to the vector instruction. This occurs when
constrainRegClass reduces to a class with less than 4 registers.
I believe LMUL8 on masked instructions triggers this since the
result can only use the v8, v16, or v24 register group as the mask
is using v0.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D98567