two changes:

1) don't try to optimize a sext or zext that is only used by a trunc, let
   the trunc get optimized first.  This avoids some pointless effort in
   some common cases since instcombine scans down a block in the first pass.
2) Change the cost model for zext elimination to consider an 'and' cheaper
   than a zext.  This allows us to do it more aggressively, and for the next
   patch to simplify the code quite a bit.

llvm-svn: 93097