Now we only support D16 folding for `image sample` instructions with a
single user: a `fptrunc` to half.
However, we can actually support D16 folding for image.sample
instructions with multiple users,
as long as each user follows the pattern of extractelement followed by
fptrunc to half.
For example:
```
%sample = call <4 x float> @llvm.amdgcn.image.sample
%e0 = extractelement <4 x float> %sample, i32 0
%h0 = fptrunc float %e0 to half
%e1 = extractelement <4 x float> %sample, i32 1
%h1 = fptrunc float %e1 to half
%e2 = extractelement <4 x float> %sample, i32 2
%h2 = fptrunc float %e2 to half
```
This change enables D16 folding for such cases and avoids generating
`v_cvt_f16_f32_e32` instructions.
0defde8e06