now that fp reg kill insertion stuff happens as a separate

pass after isel instead of being interlaced with it, we can
trust that all the code for a function has been isel'd before
it is run.

The practical impact of this is that we can scan for machine
instr phis instead of doing a fuzzy match on the LLVM BB for
phi nodes.  Doing the fuzzy match required knowing when isel
would produce an fp reg stack phi which was gross.  It was
also wrong in cases where select got lowered to a branch
tree because cmovs aren't available (PR6828).

Just do the scan on machine phis which is simpler, faster
and more correct.  This fixes PR6828.

llvm-svn: 104333