C++-style initialization of vectors (`int2 x{0,1}`) does not work, but compiles.

Description
C++-style initialization of vectors, e.g. int2 x{0,1}; appears to generate OpUndef in the SPIR-V output.
One effect that i have observed from this is that any comparisons with data from that vector are eval'ed to false.

Steps to Reproduce
DXC 1.8, with the following shader (godbolt link):

// The entry point and target profile are needed to compile this example:
// -T ps_6_6 -E PSMain

struct PSInput
{
    float4 position : SV_Position;
    float4 color    : COLOR0;
    float x : COLOR1;
    float y : COLOR2;
};


float4 PSMain(PSInput input) : SV_Target0
{
  int2 a{2, 1};
  return a.x == 2 ? float4(1.0,1.0,1.0,1.0) : float4(0.0,0.0,0.0,0.0);
}

Actual Behavior
OpUndef in generated spir-v and erratic behavior described above

I'd expect one of two outcomes:

• this syntax to mean initialization and work same way as in c++
• compile error.

Environment

• DXC version 1.8
• Host Operating System any

Start with the DXC 1.8 reproducer at the linked Godbolt entry point, using PSMain and inspecting the generated SPIR-V for OpUndef. Trace how brace initialization of int2 is parsed and lowered, then add coverage showing whether the syntax is supported or rejected, with the result matching one of the issue's stated expectations.