[IR] Add getAccessType to Instruction

There are multiple places in the code where the type of memory being accessed from an instruction needs to be obtained, including an upcoming patch to improve GEP cost modeling. This deduplicates the logic between them. It's not strictly NFC as EarlyCSE/LoopStrengthReduce may catch more intrinsics now.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D150583