semantics: Index out of bounds error when asserts are enabled

Description

Seems like this issue is debug/assert only, so not showing in the release nor godbolt.

struct SI {
  float a : A3;
  float b : A4;
};

struct S {
  float a : A;
  SI    b : A;
};

[shader("pixel")]
float main(float a : A, S s, float b : A1) : SV_Target {
  return a + s.a + s.b.a + s.b.b + b;
}

dxc -T ps_6_0 -E main /tmp/a.hlsl
arg index out of boundError: assert(false && "arg index out of bound")
File:
lib/Transforms/Scalar/ScalarReplAggregatesHLSL.cpp(4782)
Func:	AllocateSemanticIndex
Illegal instruction (core dumped)

• DXC version: f94396ddffa8562a00d64a1db58d3f73f33b655a
• Host Operating System: Linux

Reproduce the failure with the provided HLSL source and dxc command, then inspect lib/Transforms/Scalar/ScalarReplAggregatesHLSL.cpp around line 4782, where AllocateSemanticIndex reports the out-of-bounds assertion. Done means the reproducer no longer aborts when assertions are enabled and its shader compilation completes as expected.