[MachineCSE][MachinePRE] Avoid hoisting code from code regions into hot BBs.

[MachineCSE][MachinePRE] Avoid hoisting code from code regions into hot BBs.

Summary:
Current PRE hoists common computations into
CMBB = DT->findNearestCommonDominator(MBB, MBB1).
However, if CMBB is in a hot loop body, we might get performance
degradation.

Differential Revision: https://reviews.llvm.org/D64394

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366570 91177308-0d34-0410-b5e6-96231b3b80d8