[FastISel][X86] Add support for cvttss2si/cvttsd2si intrinsics.

This adds support for the cvttss2si/cvttsd2si intrinsics. Preceding
insertelement instructions are folded into the conversion instruction (if
possible).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210870 91177308-0d34-0410-b5e6-96231b3b80d8