Cache size of PassVector to speed up getNumContainedPasses().

getNumContainedPasses() used to compute the size of the vector on demand. It is
called repeated in loops (such as runOnFunction()) and it can be updated while
inside the loop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167759 91177308-0d34-0410-b5e6-96231b3b80d8