SDAG: Avoid a large, usually empty SmallVector in a recursive function
This SmallVector is using up 128 bytes on the stack every time despite
almost always being empty[1], and since this function can recurse quite
deeply that adds up to a lot of overhead. We've seen this run afoul of
ulimits in some cases with ASAN on.
Replacing the SmallVector with a std::vector trades an occasional heap
allocation for vastly less stack usage.
[1]: I gathered some stats on an internal test suite and the vector
was non-empty in only 45,000 of 10,000,000 calls to this function.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288441
91177308-0d34-0410-b5e6-
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