[AMDGPU] Fix the gfx10 scheduling model for f32 conversions

Summary:
As far as I can tell on gfx10 conversions to/from f32 (that are not
converting f32 to/from f64) are full rate instructions, but they were
marked as quarter rate instructions.

I have fixed this for gfx10 only. I assume the scheduling model was
correct for older architectures, though I don't have any documentation
handy to confirm that.

Reviewers: rampitec, arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75392