swift234

Fix a bug in the code that removes redundant overflow checks in signed multiplication.

Previously we were removing overflow checks if a previous (or future) check
used a smaller multiplier. However, this logic is incorrect for signed integers
because it does not consider underflows (example, x * 10 vs. x * -100000).

The code now checks that the multiplier that's dominated by another check is
closer or equal to zero (the absolute value is smaller or equal to the
guarding multiplier).

Swift SVN r26550