Improve the FP stackifier to decide all on its own whether

Improve the FP stackifier to decide all on its own whether
an instruction kills a register or not.  This is cheap and
easy to do now that instructions record this on their flags,
and this eliminates the second pass of LiveVariables from the
x86 backend.  This speeds up a release llc by ~2.5%.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45955 91177308-0d34-0410-b5e6-96231b3b80d8