[GlobalOpt] Perform store->dominated load forwarding for stored once globals

The initial land incorrectly optimized forwarding non-Constants in non-nosync/norecurse functions. Bail on non-Constants since norecurse should cause global -> alloca promotion anyway.

The initial land also incorrectly assumed that StoredOnceStore was the only store to the global, but it actually means that only one value other than the global initializer is stored. Add a check that there's only one store.

Compile time tracker:
https://llvm-compile-time-tracker.com/compare.php?from=c80b88ee29f34078d2149de94e27600093e6c7c0&to=ef2c2b7772424b6861a75e794f3c31b45167304a&stat=instructions

Reviewed By: nikic, asbirlea, jdoerfert

Differential Revision: https://reviews.llvm.org/D128128