Add the ability to use a direct .ll test file instead of .c/.cpp.
Add an fcmp A/B execution test, and fix the bug that it revealed.

The "fcmp une" instruction was lowered incorrectly.  This is fixed,
along with the associated lit tests.

The execution test makes all comparisons out of this set of
"interesting" values:

- negative infinity
- zero
- FLT_MIN / DBL_MIN
- 10
- FLT_MAX / DBL_MAX
- positive infinity
- NaN