[ScopDetection] Check for already known required-invariant loads [NFC]

For certain test cases we spent over 50% of the scop detection time in
checking if a load is likely invariant. We can avoid most of these checks by
testing early on if a load is expected to be invariant. Doing this reduces
scop-detection time on a large benchmark from 52 seconds to just 25 seconds.

No functional change is expected.

llvm-svn: 302134