[ValueTracking] Avoid redundant known bits calculation in computeOverflowForSignedAdd()

We're already computing the known bits of the operands here. If the
known bits of the operands can determine the sign bit of the result,
we'll already catch this in signedAddMayOverflow(). The only other
way (and as the comment already indicates) we'll get new information
from computing known bits on the whole add, is if there's an assumption
on it.

As such, we change the code to only compute known bits from assumptions,
instead of computing full known bits on the add (which would unnecessarily
recompute the known bits of the operands as well).

Differential Revision: https://reviews.llvm.org/D59473

llvm-svn: 356785