[Sema] Promote compound assignment exprs. with fp16 LHS and int. RHS.

We catch most of the various other __fp16 implicit conversions to
float, but not this one:

  __fp16 a;
  int i;
  ...
  a += i;

For which we used to generate something 'fun' like:

  %conv = sitofp i32 %i to float
  %1 = tail call i16 @llvm.convert.to.fp16.f32(float %conv)
  %add = add i16 %0, %1

Instead, when we have an __fp16 LHS and an integer RHS, we should
use float as the result type.

While there, add a bunch of missing tests for mixed
__fp16/integer expressions.

llvm-svn: 238625