Re-reapply r221924: "[GVN] Perform Scalar PRE on gep indices that feed loads before

doing Load PRE"

It's not really expected to stick around, last time it provoked a weird LTO
build failure that I can't reproduce now, and the bot logs are long gone. I'll
re-revert it if the failures recur.

Original description: Perform Scalar PRE on gep indices that feed loads before
doing Load PRE.

llvm-svn: 225536