[X86] Remove X86ISD::FILD_FLAG and stop gluing nodes together.

Summary:
I think whatever problem the gluing was fixing has long since been fixed. We don't have any of the restrictions on FP stack stuff that existed back when this was first added.

I had to change which type we use for FILD in BuildFILD when X86 was enabled because most of the isel patterns block f32/f64 instructions when SSE1/SSE2 are enabled. So I needed to use the f80 pattern, but this shouldn't have an effect the generated code since there is only one FILD instruction anyway. We already use f80 explicitly in other other places.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: andrew.w.kaylor, scanon, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72805