[X86] Directly call EmitTest in two places instead of creating a null constant...

[X86] Directly call EmitTest in two places instead of creating a null constant and calling EmitCmp. NFCI

EmitCmp will just immediately call EmitTest and discard the null
constant only to have EmitTest create it again if it doesn't fold.

So just skip all that and go directly to EmitTest.