Remove the API compatibility layer which converted add, sub, and mul

Remove the API compatibility layer which converted add, sub, and mul
to fadd, fsub, and fmul, when used with a floating-point type. LLVM
has supported the new instructions since 2.6, so it's time to get
on board.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102971 91177308-0d34-0410-b5e6-96231b3b80d8