[X86] Remove the scalar intrinsics for fadd/fsub/fdiv/fmul

Summary: These intrinsics have been unused for clang for a while. This patch removes them. We auto upgrade them to extractelements, a scalar operation and then an insertelement. This matches the sequence used by clangs intrinsic file.

Reviewers: zvi, delena, RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26660

llvm-svn: 287083