SLPVectorizer: limit the scheduling region size per basic block.

SLPVectorizer: limit the scheduling region size per basic block.

Usually large blocks are not a problem. But if a large block (> 10k instructions)
contains many (potential) chains of vector instructions, and those chains are
spread over a wide range of instructions, then scheduling becomes a compile time problem.
This change introduces a limit for the accumulate scheduling region size of a block.
For real-world functions this limit will never be exceeded (it's about 10x larger than
the maximum value seen in the test-suite and external test suite).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248917 91177308-0d34-0410-b5e6-96231b3b80d8