LiveVariables::VarInfo contains an AliveBlocks BitVector, which has as many

LiveVariables::VarInfo contains an AliveBlocks BitVector, which has as many
entries as there are basic blocks in the function.  LiveVariables::getVarInfo
creates a VarInfo struct for every register in the function, leading to
quadratic space use.  This patch changes the BitVector to a SparseBitVector,
which doesn't help the worst-case memory use but does reduce the actual use in
very long functions with short-lived variables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72426 91177308-0d34-0410-b5e6-96231b3b80d8