[RISCV] Don't relax policy to ta when vmerge's VL shrinks during folding

When folding a vmerge into its operands, if the resulting VL is smaller than
what the vmerge had originally then what was previously in its body then gets
moved to the tail. In that case, we can't relax the tail policy to agnostic
when the merge operand is undefined, since we need to preserve these elements
past the new VL.

Fixes https://github.com/llvm/llvm-project/issues/64754

Reviewed By: craig.topper, reames

Differential Revision: https://reviews.llvm.org/D158161