Remove redundant uav load/store caused by partial update or copyin/out

[python3kgae]

For the repro, only u[i].b need to be load and store.

struct S {

float4x4 a[10]; float4 b; };

RWStructuredBuffer< S > u;

void foo(inout S s) {

s.b += sin(s.b);

}

float4 main(uint i:I) : SV_Target { foo(u[i]); return u[i].b; }

[AppVeyorBot]

:white_check_mark: Build DirectXShaderCompiler 1.0.1056 completed (commit https://github.com/microsoft/DirectXShaderCompiler/commit/bbbfe35ec8 by @)

[AppVeyorBot]

:white_check_mark: Build DirectXShaderCompiler 1.0.1060 completed (commit https://github.com/microsoft/DirectXShaderCompiler/commit/3aaf47c156 by @)