[clang] [AMDGPU] Stop coercing structs with FP and int fields to integer arrays (PR #185083)

[via cfe-commits]

addmisol wrote:

> @arsenm and @addmisol I seems seeing some behavior change with this PR. The small struct with fp16 type used to be packed, no longer gets packed.

The behavior change (fp16 structs no longer packed to i32) is intentional - we preserve float types now.

However, I understand the concern about packing efficiency.. A potential solution would be to pack uniform float structs into vector types instead(if i am correct..):

{__fp16, __fp16}  to  <2 x half>    (instead of i32 or {half, half})
{float, float}  to  <2 x float>   (instead of [2 x i32] or {float, float})
 
This would:
-  Preserve float type information
-  Pack into a single value/register
-  Enable SIMD optimizations

cc @arsenm 

https://github.com/llvm/llvm-project/pull/185083