[InstCombine] Freeze other uses of frozen value

If there is a freeze %x, we currently replace all other uses of %x
with freeze %x -- as long as they are dominated by the freeze
instruction. This patch extends this behavior to cases where we
did not originally dominate the use by moving the freeze
instruction directly after the definition of the frozen value.

The motivation can be seen in test @combine_and_after_freezing_uses:
Canonicalizing everything to freeze %x allows folds that are based
on value identity (i.e. same operand occurring in two places) to
trigger. This also covers the case from D125248.

Differential Revision: https://reviews.llvm.org/D125321