[GVN] Assign new value number to calls reading memory, if there is no MemDep info.

Currently we assign the same value number to two calls reading the same
memory location if we do not have MemoryDependence info. Without MemDep
Info we cannot guarantee that there is no store between the two calls, so we
have to assign a new number to the second call.

It also adds a new option EnableMemDep to enable/disable running
MemoryDependenceAnalysis and also renamed NoLoads to NoMemDepAnalysis to
be more explicit what it does. As it also impacts calls that read memory,
NoLoads is a bit confusing.

Reviewers: efriedma, sebpop, john.brawn, wmi

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D50893

llvm-svn: 340319