Revert "[BasicAA] Handle two unknown sizes for GEPs"

Temporarily revert commit 8b1c4e31.

After 8b1c4e31 the compile-time for `MultiSource/Benchmarks/MiBench/consumer-lame`
dramatically increases with -O3 & LTO, causing issues for builders with
that configuration.

I filed PR48553 with a smallish reproducer that shows a 10-100x compile
time increase.