change the canonical form of "cond ? -1 : 0" to be

change the canonical form of "cond ? -1 : 0" to be
"sext cond" instead of a select.  This simplifies some instcombine
code, matches the policy for zext (cond ? 1 : 0 -> zext), and allows
us to generate better code for a testcase on ppc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94339 91177308-0d34-0410-b5e6-96231b3b80d8