add the popcount instruction and support this in the isel

the primary user of this will probably end up being find-first-set-bit/find-
last-set-bit, which i'll get around to...


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@21860 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed