Introduce -unroll-partial-threshold to separate PartialThreshold from Threshold in loop unorller.

Summary: Partial unrolling should have separate threshold with full unrolling.

Reviewers: efriedma, mzolotukhin

Reviewed By: efriedma, mzolotukhin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28831

llvm-svn: 292293