[llvm] [RISCV][VLOPT] Be more permissive when there is a non-undef passthru (PR #120651)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

Closing as won't do because of @preames point about TA requires specifically either prior value or -1.

https://github.com/llvm/llvm-project/pull/120651