[X86] Re-enable lowerUINT_TO_FP_vXi32 under fast-math by using an FSUB instead of an FADD.

Summary:
We previously disabled this under fast math due to aggressive
reassociation by the machine combiner. But I think we can work
around this by using a FSUB instead of FADD for the first
operation.

This matches the similar algorithm we do for uint_to_fp i64->f64
in TargetLowering::expandUINT_TO_FP. If reassociation hasn't
been a problem for that, hopefully its not a problem here.

Reviewers: RKSimon, spatel, scanon

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71968