Fix Issue 14785 - Some corner cases are not handled properly by core.checkedint.

[tsbockman]

This pull request fixes druntime Issue #14785.

/** Test cases that fail without this PR **/

import core.checkedint;
void main(string[] args)
{
    bool overflow = false;
    assert(subs(-1L, long.min, overflow) == long.max);
    assert(!overflow); // Assertion failure

    overflow = false;
    assert(muls(-1L, long.min, overflow) == long.min); // Floating Point Exception!
    assert(overflow);
}

[DmitryOlshansky]

Please squash the commits, really no point in having a separate entry for the whitespace fix

[tsbockman]

@DmitryOlshansky OK. I fixed the commit history.

Any non-whitespace related comments?

[burner]

LGTM

[DmitryOlshansky]

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