PR878: Instead of calculating the vreg to PHI use count everytime we get

PR878: Instead of calculating the vreg to PHI use count everytime we get
a function, do it up front in linear time (going through all of the
instructions once). We create a map out of them. Then it's no problem to
use the information in it during elimination...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30624 91177308-0d34-0410-b5e6-96231b3b80d8