[PlaceSafepoints] Make the width of a counted loop settable.

[PlaceSafepoints] Make the width of a counted loop settable.

Summary:
This change lets a `PlaceSafepoints` client change how wide the trip
count of a loop has to be for the loop to be considerd "counted", via
`CountedLoopTripWidth`.  It also removes the boolean `SkipCounted` flag
and the `upperTripBound` constant -- we can get the old behavior of
`SkipCounted` == `false` by setting `CountedLoopTripWidth` to `13` (2 ^
13 == 8192).

Reviewers: reames

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D12789

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247656 91177308-0d34-0410-b5e6-96231b3b80d8