[X86] Check if call is indirect before emitting NT_CALL

The notrack prefix is a relaxation of CET policies which makes it possible to indirectly call targets which do not have an ENDBR instruction in the landing address. To emit a call with this prefix, the special attribute "nocf_check" is used. When used as a function attribute, a CallInst targeting the respective function will return true for the method "doesNoCfCheck()", no matter if it is a direct call (and such should remain like this, as the information that the to-be-called function won't perform control-flow checks is useful in other contexts). Yet, when emitting an X86ISD::NT_CALL, the respective CallInst should be verified for its indirection, allowing that the prefixed calls are only emitted in the right situations.

Update the respective testing unit to also verify for direct calls to functions with ''nocf_check'' attributes.

The bug can also be reproduced through compiling the following C code using the -fcf-protection=full flag.

int __attribute__((nocf_check)) foo(int a) {};

int main() {
  foo(42);
}

Differential Revision: https://reviews.llvm.org/D87320

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