[HLSL][SPIRV] Remove unecessary `STI.isShader()` check that avoid `OpConstantNull`

Currently, to match DXC existing behaviour, there are a few cases where `STI.isShader()` is used to decide if a null constant should be used when lowering to SPIRV.

https://github.com/llvm/llvm-project/blob/f04b27125ad150f3ca7488272cddc3a6341cbf15/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp#L4159

After an internal discussion, we decided to NOT replicate this pattern in clang. So such checks can be removed. For example, the following code:

```spirv
%[[#const_i16_0:]] = OpConstant %[[#int_16]] 0
OpConstantComposite %[[#vec4_16:]] %[[#const_i16_0:]] %[[#const_i16_0:]] %[[#const_i16_0:]] %[[#const_i16_0:]]
```
Should lower to:

```spirv
%[[#const_i16_0:]] = OpConstantNull %[[#int_16]]
```