[X86][WIP] Change precision control to FP80 during u64->fp32 conversion on Windows.

This is an alternative to D141074 to fix the problem by adjusting
the precision control dynamically.

This isn't quite complete yet. I want to support fadd with an load
folded into it too. That's the code we will usually generate.

Posting for early review so we can do some testing of this solution.

Differential Revision: https://reviews.llvm.org/D142178