[Inliner] Pass nullptr for the ORE param of getInlineCost if RemarkEnabled

[Inliner] Pass nullptr for the ORE param of getInlineCost if RemarkEnabled
is false.

Right now for inliner and partial inliner, we always pass the address of a
valid ORE object to getInlineCost even if RemarkEnabled is false because of
no -Rpass is specified. Since ComputeFullInlineCost will be set to true if
ORE is non-null in getInlineCost, this introduces the problem that in
getInlineCost we cannot return early even if we already know the cost is
definitely higher than the threshold. It is a general problem for compile
time.

This patch fixes that by pass nullptr as the ORE argument if RemarkEnabled is
false.

Differential Revision: https://reviews.llvm.org/D58399

llvm-svn: 354542