[InstCombine] Use a cl::opt to control calls to getOrEnforceKnownAlignment in...

[InstCombine] Use a cl::opt to control calls to getOrEnforceKnownAlignment in LoadInst and StoreInst

This is in preparation for the InferAlignment pass which handles
inferring alignment for instructions separately. It is better to handle
this as a separate pass as inferring alignment is quite costly, and
InstCombine running multiple times in the pass pipeline makes it even
more so.

Differential Revision: https://reviews.llvm.org/D158527