Shrink wrapping in PEI:
- reduces _static_ callee saved register spills
  and restores similar to Chow's original algorithm.
- iterative implementation with simple heuristic
  limits to mitigate compile time impact.
- handles placing spills/restores for multi-entry,
  multi-exit regions in the Machine CFG without
  splitting edges.
- passes test-suite in LLCBETA mode.

Added contains() method to ADT/SparseBitVector.

llvm-svn: 71438