[DXIL] implement `trunc` lowering

[farzonl]

https://godbolt.org/z/qYMTWdW3r

Simple change just need to add mapping from Intrinsic::trunc to dixil op 29

like so:

@dx.op.unary.f16(i32 29, half %x) 
@dx.op.unary.f32(i32 29, float %x)

[hekota]

I can take a look at this.