Make processing @llvm.assume more efficient - Add affected values to the assumption cache

Here's my second try at making @llvm.assume processing more efficient. My
previous attempt, which leveraged operand bundles, r289755, didn't end up
working: it did make assume processing more efficient but eliminating the
assumption cache made ephemeral value computation too expensive. This is a
more-targeted change. We'll keep the assumption cache, but extend it to keep a
map of affected values (i.e. values about which an assumption might provide
some information) to the corresponding assumption intrinsics. This allows
ValueTracking and LVI to find assumptions relevant to the value being queried
without scanning all assumptions in the function. The fact that ValueTracking
started doing O(number of assumptions in the function) work, for every
known-bits query, has become prohibitively expensive in some cases.

As discussed during the review, this is a pragmatic fix that, longer term, will
likely be replaced by a more-principled solution (perhaps based on an extended
SSA form).

Differential Revision: https://reviews.llvm.org/D28459

llvm-svn: 291671