Recommit : Use the basic cost if a GEP is not used as addressing mode

Recommitting r314517 with the fix for handling ConstantExpr.

Original commit message:
  Currently, getGEPCost() returns TCC_FREE whenever a GEP is a legal addressing
  mode in the target. However, since it doesn't check its actual users, it will
  return FREE even in cases where the GEP cannot be folded away as a part of
  actual addressing mode. For example, if an user of the GEP is a call
  instruction taking the GEP as a parameter, then the GEP may not be folded in
  isel.

llvm-svn: 314923