vec_lde and vec_expte semi-randomly produce the wrong results
Description of problem:
I found that while implementing the Altivec support for the rust [stdarch](https://github.com/rust-lang/stdarch).
Steps to reproduce:
1. Install rust nightly (e.g. using https://rustup.rs/)
2. `git clone https://github.com/rust-lang/stdarch`
3. You need to either cross compile or compile and run the tests for `crates/core_arch`.
Additional information:
Both `valgrind` and running on power9 produce the correct results