[ExpandLargeFpConvert] Fix bug in int-to-fp expansion. (#85370)

When deciding whether to perform rounding on the significand,
the generated IR was using (width - leading zeros - 1) rather
than (width - leading zeros). This is different from how the
routine in compiler-rt does it:

    int sd = srcBits - clzSrcT(a);
    int e = sd - 1;
    if (sd > dstMantDig) {

This bug means that the following code, when built on -O0:

    #include <stdio.h>

    _BitInt(233) v_1037 = 0;

    int main(void)
    {
        v_1037 = 18014398509481982wb;
        double d = v_1037;
        printf("d = %f\n", d);

        return 0;
    }

prints "d = 9007199254740992.000000", which is incorrect.
The correct result is "d = 18014398509481982.000000".