[TableGen] CodeGenDAGPatterns::GenerateVariants - use BitVector::push_back()

As mentioned on D52236, this saves 10secs in debug builds of x86 -gen-dag-isel

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342536 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed