[GVN] Don't use the eliminated load as an available value in phi construction

In ConstructSSAForLoadSet if an available value is actually the load that we're
doing SSA construction to eliminate, then we can omit it as SSAUpdate will add
in the value for the phi that will be replacing it anyway. This can result in
simpler IR which can allow further optimisation.

Differential Revision: https://reviews.llvm.org/D44160

llvm-svn: 337686