Change SCEVNAryExpr's operand array from a SmallVector to a plain

Change SCEVNAryExpr's operand array from a SmallVector to a plain
pointer and length, and allocate the arrays in ScalarEvolution's
BumpPtrAllocator, so that they get released when their owning
SCEV gets released. SCEVs are immutable, so they don't need to worry
about operand array resizing. This fixes a memory leak reported
in PR6637.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98755 91177308-0d34-0410-b5e6-96231b3b80d8