cgen: better code generation for .noreturn calls

[zerbina]

Summary

Improve C code generation for .noreturn calls, by omitting the
unnecessary error flag check for them.

Details

If a procedure only returns due to exceptional unwinding, the
if(NIM_UNLIKELY(*nimErr_)) guard guarding the unwinding is
unnecessary, since it's guaranteed that the error flag is set (all
exits of the procedure are due to exceptional unwinding).

Unconditionally jumping to the error handling target makes it clear
to the C compiler that control-flow doesn't continue normally after
the call

Other improvements

In addition, the canRaiseConservative usage is unnecessary too, since
a cnkCheckedCall already signals that the call can raise. The
procedure has no more users and is thus removed.

Finally, for compilers that support it, a call to a .noreturn
procedure that doesn't raise is now followed by an __assume(0)
statement, communicating to the C compiler that control-flow never
returns to the caller.

[zerbina]

For an example of how code generation improves, consider the following:

proc test(num: int) =
  let val =
    if num == 2: 1
    elif num == 4: 2
    else: unreachable() # call to a `.noreturn` procedure

  if val >= 1:
    echo "here"

The produced C code was roughly equivalent to the following:

int* errorFlag;
void unreachable();

void test(int num) {
    int val = 0;
    if (num == 2) {
        val = 1;
    } else if(num == 4) {
        val = 2;
    } else {
        unreachable();
        if (*errorFlag) goto BeforeRet_;
    }
    if (num >= 1) {
        printf("here");
    }
    BeforeRet_:
    return;
}

(optimized assembler)

After the PR, the if(*errorFlag) guard is gone. As can be seen here, an optimizing C compiler (gcc, in this case) is then able omit the code path for the (impossible) *errorFlag == 0 case.

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• a preparation for lowering the runtime check magics with a MIR pass
• marking procedures as .noreturn previously didn't improve code generation; now it does