Fix side-effect detection in LLVMIRIntrinsicGen.

Previously it reported an op had side-effects iff it declared that it
didn't have any side-effects.  This had the undesirable result that
canonicalization would always delete any intrinsic calls that did memory
stores and returned void.

Reviewed By: ftynse, mehdi_amini

Differential Revision: https://reviews.llvm.org/D96369