HLSL: Zero-init OpUndef Phi incomings to avoid uninitialized reads

[xen2]

When an OpPhi has OpUndef on an incoming edge, SPIRV-Cross emitted a read of the source variable. If that source was uninitialized too, FXC rejects the HLSL with X4555 or X4000.

OpUndef allows any value per spec, so we substitute zero:

• flush_phi: emit lhs = 0 when the Phi source is OpUndef.
• Loop variable declarations: add = 0 when static_expression is OpUndef, via a new undef_loop_variable_initializer_suffix helper.

This 2nd version also make sure to not affect GLSL which doesn't need it.

Test

Added shaders-hlsl-no-opt/asm/comp/phi-undef-loop.asm.comp — a minimal SPIR-V loop with OpUndef as the preheader Phi incoming.

Without the patch, the generated HLSL is:

uint _19; // not set
for (uint _22 = 0u; _22 < 10u; _19++, _22++) // error X4555: cannot use casts on l-values (due to _19++ which is not set)
{
    _4.Store(0, _19);
}

With the patch, _19 is declared as uint _19 = 0u; and FXC accepts it.

[HansKristian-Work]

Seems reasonable.