Revert "[RISCV] Avoid changing etype for splat of 0 or -1"

This reverts commit 755c84c6.  A bug was reported on the original review thread (https://reviews.llvm.org/D128006), and on inspection this patch is simply wrong.  It needs to be checking for VLInBytes, not MaxVL.  These happen to be the same when using AVL=VLMAX (which is quite common), but this does not fold when AVL != VLMAX.