Fix pad_to doesn't silence error 131

[siliconvoodoo]

In case of such a program as input:

ShaderResourceGroupSemantic ExampleBinding { FrequencyId = 0; };
ShaderResourceGroup ExampleSRG : ExampleBinding
{
    float3x3 mat;
    bool b;
};

We get the result

IR error #131: Detected potential alignment issues related with DXC flag '-fvk-use-dx-layout'. Alternatively you can use the option '--no-alignment-validation' to void this error

• A 'float2' variable should be added before the variable 'b'

It seems natural that we can either follow the recommendation, or use the [[pad_to()]] attribute as such:

    float3x3 mat;
    [[pad_to(44)]]
    bool b;

Doing so in conjunction with --no-alignment-validation did result in a float2 being inserted so the error message should go away. But it didn't. This PR fixes that.

Secondly, there was a crash in case of this input:

    float2x2 mat;
    [[pad_to(36)]]
    bool b;

[siliconvoodoo]

ping @galibzon for review?