[LICM] Cap the clobbering calls in LICM.

Summary:
Unlimitted number of calls to getClobberingAccess can lead to high
compile times in pathological cases.
Switching EnableLicmCap flag from bool to int, and enabling to default 100.
(tested to be appropriate for current bechmarks)
We can revisit this value when enabling MemorySSA.

Reviewers: sanjoy, chandlerc, george.burgess.iv

Subscribers: jlebar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57968

llvm-svn: 353897