Move the code that inserts X87 FP_REG_KILL instructions from a

Move the code that inserts X87 FP_REG_KILL instructions from a
special-purpose hook to a new pass. Also, add check to see if any
x87 virtual registers are used, to avoid doing any work in the
common case that no x87 code is needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59190 91177308-0d34-0410-b5e6-96231b3b80d8